[autotests] Enforce OpenGL compositing

As expected in a025791d7b the tests
fail on the CI system  due to llvmpipe. Let's try whether enforcing
O2 makes them run, if not update to switch to Q is following.

autotests/wayland/kwin_wayland_test.cpp

@@ -42,6 +42,7 @@ static void readDisplay(int pipe);
 WaylandTestApplication::WaylandTestApplication(int &argc, char **argv)
     : Application(OperationModeXwayland, argc, argv)
 {
+    qputenv("KWIN_COMPOSE", QByteArrayLiteral("O2"));
     WaylandServer *server = WaylandServer::create(this);
     QPluginLoader loader(QStringLiteral(KWINBACKENDPATH));
     loader.instance()->setParent(server);