Make ZopeTestInSubProcess a RunTest thing, rather than a TestCase mixin

[Jonathan Lange]

ZopeTestInSubProcess runs its tests in subprocesses. It's great, but we could do the same thing using testtools RunTest mechanism, so that tests would be written as::

  class TestFoo(TestCase):
    run_tests_with = ZopeSubProcessRunner

Rather than::

  class TestFoo(ZopeTestInSubProcess, TestCase):

The main advantage to this would be that we could attach the stdout and stderr of the child process to the main test case and have it appear in test failures, which would be handy for debugging.

[Colin Watson (cjwatson)]

I hadn't seen this bug at the time, but I fixed this last year in https://code.launchpad.net/~cjwatson/launchpad/+git/launchpad/+merge/387645.