sb-introspect self-tests fail when sb-source-locations disabled

[Tomas Hlavaty]

on ubuntu:

hlavaty@u14:~/sbcl$ cat customize-target-features.lisp
(lambda (features)
  (flet ((enable (x)
                 (pushnew x features))
         (disable (x)
                  (setf features (remove x features))))
        (disable :sb-source-locations)
        ))

WARNING! Some of the contrib modules did not build successfully or pass
their self-tests. Failed contribs:"
  sb-introspect
Command exited with non-zero status 1

3 out of 96 total tests failed: SB-INTROSPECT-TEST::FIND-SOURCE-STUFF.4,
   SB-INTROSPECT-TEST::FIND-SOURCE-STUFF.12,
   SB-INTROSPECT-TEST::FIND-SOURCE-STUFF.24.
WARNING: ignoring expected failures in test-op
Unhandled SIMPLE-ERROR in thread #<SB-THREAD:THREAD "main thread" RUNNING
                                    {1002CCEE03}>:
  test-op failed with unexpected failures