TestSuite: call flightgear::shutdownQtApp() from fgtest::shutdownTestGlobals()

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

test_suite/helpers/globals.cxx

@@ -3,6 +3,10 @@
 
 #include "globals.hxx"
 
+#if defined(HAVE_QT)
+#include <GUI/QtLauncher.hxx>
+#endif
+
 #include <Main/globals.hxx>
 #include <Main/options.hxx>
 
@@ -93,6 +97,12 @@ namespace fgtest
 
   void shutdownTestGlobals()
   {
+    // The QApplication instance must be destroyed before exit() begins, see
+    // <https://bugreports.qt.io/browse/QTBUG-48709> (otherwise, segfault).
+#if defined(HAVE_QT)
+    flightgear::shutdownQtApp();
+#endif
+
     delete globals;
   }
 } // of namespace fgtest