uitest: add missing solar mutex guard

Change-Id: Ic86e7e75ac8667b32227683c5b48731831203a0f
Reviewed-on: https://gerrit.libreoffice.org/31994
Tested-by: Jenkins <ci@libreoffice.org>
Reviewed-by: Markus Mohrhard <markus.mohrhard@googlemail.com>

vcl/source/uitest/uno/uitest_uno.cxx

@@ -70,6 +70,7 @@ void SAL_CALL UITestUnoObj::executeCommand(const OUString& rCommand)
 css::uno::Reference<css::ui::test::XUIObject> SAL_CALL UITestUnoObj::getTopFocusWindow()
     throw (css::uno::RuntimeException, std::exception)
 {
+    SolarMutexGuard aGuard;
     std::unique_ptr<UIObject> pObj = UITest::getFocusTopWindow();
     return new UIObjectUnoObj(std::move(pObj));
 }