Some of the unit tests defined with the new test infrastructure[1]
initialize the QApplication; for these tests, it is necessary to ensure
that the QApplication instance is destroyed before exit() begins,
otherwise we get a segfault when run_test_suite terminates (see [2]).
In order to prevent this segfault from happening, call
flightgear::shutdownQtApp() unconditionally from
fgtest::shutdownTestGlobals() as long as Qt support is compiled in
(flightgear::shutdownQtApp() is safe to call even if the QApplication
hasn't been initialized).
[1] Currently, the three PosInitTests::* tests.
[2] https://bugreports.qt.io/browse/QTBUG-48709
The exceptions that occur after this call can be handled by CppUnit, but the
exit() results in the test suite silently exiting without restoring stdout and
stderr.
