Add environment variable support for testability library use.

Add the option to load the testability library based on whether
an environment variable QT_LOAD_TESTABILITY is set, in addition
to the current "-testability" flag.

This improves the use of testability library together with the
autopilot functional testing tool.

Task-number: QTBUG-32974
Change-Id: I6abf2c004cbff2ce0aff44e24a99bcc8188a52ae
Reviewed-by: Olivier Goffart (Woboq GmbH) <ogoffart@woboq.com>

src/gui/kernel/qguiapplication.cpp

@@ -1329,6 +1329,9 @@ void QGuiApplicationPrivate::init()
 #endif
 
 #ifndef QT_NO_LIBRARY
+    if (qEnvironmentVariableIntValue("QT_LOAD_TESTABILITY") > 0)
+        loadTestability = true;
+
     if (loadTestability) {
         QLibrary testLib(QStringLiteral("qttestability"));
         if (testLib.load()) {