CMake: Fix tst_qapplication to respect blacklists in CMake builds

CMake builds are special and need to know the path to the source dir.
This is handled automatically by QTEST_MAIN, but tst_qapplication
doesn't use QTEST_MAIN. Thus we need to call
QTEST_SET_MAIN_SOURCE_PATH manually.

Task-number: QTBUG-87137
Change-Id: Ib2c461f0da0a3d9a2f571f37476b750a606065f7
Reviewed-by: Volker Hilsheimer <volker.hilsheimer@qt.io>

tests/auto/widgets/kernel/qapplication/tst_qapplication.cpp

@@ -2551,6 +2551,7 @@ int main(int argc, char *argv[])
 {
     tst_QApplication tc;
     argv0 = argv[0];
+    QTEST_SET_MAIN_SOURCE_PATH
     return QTest::qExec(&tc, argc, argv);
 }