tst_qmutex autotest is unstable on OSX 10.7

tst_qmutex appears to be slightly unstable on OSX 10.7, occasionally giving different results in sequential runs or hanging on exit.

I scraped the last 317 CI runs (2500 through 2817) and found:

• http://testresults.qt-project.org/ci/QtBase_master_Integration/build_02506/macx-clang_OSX_10.7/log.txt.gz - test gives different results on two sequential runs
• http://testresults.qt-project.org/ci/QtBase_master_Integration/build_02817/macx-clang_OSX_10.7/log.txt.gz - hung on exit

Also, the test will occasionally output "QMutex: destroying locked mutex" one or more times at exit.