I think the general pattern is that WPushButtonTest calls qWait which runs the Qt event loop, processing events that have been queued from other tests. 

Here's an example where GlobalTrackCache::~GlobalTrackCache is run from a deleteLater event:

[ RUN      ] WPushButtonTest.QuickPressNoLatchTest
Loading resources from  "/home/rryan/Code/mixxx/res/"
ControlDoublePrivate::getControl returning NULL for ( "[Controls]" , "touch_shift" )
ControlDoublePrivate::getControl returning NULL for ( "[Controls]" , "touch_shift" )
ControlDoublePrivate::getControl returning NULL for ( "[Master]" , "num_decks" )

Thread 1 "mixxx-test" received signal SIGSEGV, Segmentation fault.
0x0000000000a8ed0c in std::_Sp_counted_deleter<LibraryTest*, LibraryTest::LibraryTest()::{lambda(GlobalTrackCacheSaver*)#1}, std::allocator<void>, (__gnu_cxx::_Lock_policy)2>::_M_dispose() (this=0x1a16660)
    at /usr/bin/../lib/gcc/x86_64-linux-gnu/5.4.0/../../../../include/c++/5.4.0/bits/shared_ptr_base.h:466
466           { _M_impl._M_del()(_M_impl._M_ptr); }
(gdb) bt
#0  0x0000000000a8ed0c in std::_Sp_counted_deleter<LibraryTest*, LibraryTest::LibraryTest()::{lambda(GlobalTrackCacheSaver*)#1}, std::allocator<void>, (__gnu_cxx::_Lock_policy)2>::_M_dispose() (this=0x1a16660)
    at /usr/bin/../lib/gcc/x86_64-linux-gnu/5.4.0/../../../../include/c++/5.4.0/bits/shared_ptr_base.h:466
#1  0x000000000050e3e5 in std::_Sp_counted_base<(__gnu_cxx::_Lock_policy)2>::_M_release (this=0x1a16660) at /usr/bin/../lib/gcc/x86_64-linux-gnu/5.4.0/../../../../include/c++/5.4.0/bits/shared_ptr_base.h:150
#2  0x000000000050e39a in std::__shared_count<(__gnu_cxx::_Lock_policy)2>::~__shared_count (this=0x7fffffffd5b0) at /usr/bin/../lib/gcc/x86_64-linux-gnu/5.4.0/../../../../include/c++/5.4.0/bits/shared_ptr_base.h:659
#3  0x00000000005d39a9 in std::__shared_ptr<GlobalTrackCacheSaver, (__gnu_cxx::_Lock_policy)2>::~__shared_ptr (this=0x7fffffffd5a8) at /usr/bin/../lib/gcc/x86_64-linux-gnu/5.4.0/../../../../include/c++/5.4.0/bits/shared_ptr_base.h:925
#4  0x0000000000cbd326 in std::__shared_ptr<GlobalTrackCacheSaver, (__gnu_cxx::_Lock_policy)2>::reset (this=0x1a88078) at /usr/bin/../lib/gcc/x86_64-linux-gnu/5.4.0/../../../../include/c++/5.4.0/bits/shared_ptr_base.h:1022
#5  0x0000000000cb79f4 in GlobalTrackCache::deactivate (this=0x1a88060) at src/track/globaltrackcache.cpp:349
#6  0x0000000000cba7b6 in GlobalTrackCache::~GlobalTrackCache (this=0x1a88060) at src/track/globaltrackcache.cpp:259
#7  0x0000000000cba889 in GlobalTrackCache::~GlobalTrackCache (this=0x1a88060) at src/track/globaltrackcache.cpp:258
#8  0x00007ffff5978c20 in QObject::event(QEvent*) () from /usr/lib/x86_64-linux-gnu/libQt5Core.so.5
#9  0x00007ffff62ba05c in QApplicationPrivate::notify_helper(QObject*, QEvent*) () from /usr/lib/x86_64-linux-gnu/libQt5Widgets.so.5
#10 0x00007ffff62bf516 in QApplication::notify(QObject*, QEvent*) () from /usr/lib/x86_64-linux-gnu/libQt5Widgets.so.5
#11 0x00007ffff594938b in QCoreApplication::notifyInternal(QObject*, QEvent*) () from /usr/lib/x86_64-linux-gnu/libQt5Core.so.5
#12 0x00007ffff594b786 in QCoreApplicationPrivate::sendPostedEvents(QObject*, int, QThreadData*) () from /usr/lib/x86_64-linux-gnu/libQt5Core.so.5
#13 0x0000000000ca0e7d in QTest::qWait (ms=1) at /usr/include/x86_64-linux-gnu/qt5/QtTest/qtestsystem.h:59
#14 0x0000000000ca0698 in QTest::mouseEvent (action=QTest::MouseRelease, widget=0x18f77c0, button=Qt::LeftButton, stateKey=..., pos=..., delay=1) at /usr/include/x86_64-linux-gnu/qt5/QtTest/qtestmouse.h:161
#15 0x0000000000ca1f6f in QTestMouseEvent::simulate (this=0xcebfa90, w=0x18f77c0) at /usr/include/x86_64-linux-gnu/qt5/QtTest/qtestevent.h:127
#16 0x0000000000ca11a5 in QTestEventList::simulate (this=0xcf220e0, w=0x18f77c0) at /usr/include/x86_64-linux-gnu/qt5/QtTest/qtestevent.h:211
#17 0x0000000000c9fc8e in WPushButtonTest_QuickPressNoLatchTest_Test::TestBody (this=0xcf220b0) at src/test/wpushbutton_test.cpp:50
#18 0x0000000000e19caa in testing::internal::HandleSehExceptionsInMethodIfSupported<testing::Test, void> (object=0xcf220b0, method=&virtual testing::Test::TestBody(), location=0x11c1c84 "the test body")
    at lib/gtest-1.7.0/src/gtest.cc:2078
#19 0x0000000000e0aac7 in testing::internal::HandleExceptionsInMethodIfSupported<testing::Test, void> (object=0xcf220b0, method=&virtual testing::Test::TestBody(), location=0x11c1c84 "the test body") at lib/gtest-1.7.0/src/gtest.cc:2114
#20 0x0000000000df31d5 in testing::Test::Run (this=0xcf220b0) at lib/gtest-1.7.0/src/gtest.cc:2150
#21 0x0000000000df3e98 in testing::TestInfo::Run (this=0x1892aa0) at lib/gtest-1.7.0/src/gtest.cc:2326
#22 0x0000000000df4557 in testing::TestCase::Run (this=0x1892bd0) at lib/gtest-1.7.0/src/gtest.cc:2444
#23 0x0000000000dfbd11 in testing::internal::UnitTestImpl::RunAllTests (this=0x185fe40) at lib/gtest-1.7.0/src/gtest.cc:4315
#24 0x0000000000e1d28a in testing::internal::HandleSehExceptionsInMethodIfSupported<testing::internal::UnitTestImpl, bool> (object=0x185fe40,
    method=(bool (testing::internal::UnitTestImpl::*)(testing::internal::UnitTestImpl * const)) 0xdfba30 <testing::internal::UnitTestImpl::RunAllTests()>, location=0x11c2362 "auxiliary test code (environments or event listeners)")
    at lib/gtest-1.7.0/src/gtest.cc:2078
#25 0x0000000000e0ce77 in testing::internal::HandleExceptionsInMethodIfSupported<testing::internal::UnitTestImpl, bool> (object=0x185fe40,
    method=(bool (testing::internal::UnitTestImpl::*)(testing::internal::UnitTestImpl * const)) 0xdfba30 <testing::internal::UnitTestImpl::RunAllTests()>, location=0x11c2362 "auxiliary test code (environments or event listeners)")
    at lib/gtest-1.7.0/src/gtest.cc:2114
#26 0x0000000000dfb9e3 in testing::UnitTest::Run (this=0x179c730 <testing::UnitTest::GetInstance()::instance>) at lib/gtest-1.7.0/src/gtest.cc:3926
#27 0x0000000000bd2fb1 in RUN_ALL_TESTS () at lib/gtest-1.7.0/include/gtest/gtest.h:2288
#28 0x0000000000bd2f5e in main (argc=1, argv=0x7fffffffe688) at src/test/main.cpp:33
(gdb)