Improve timer dependent tests.

tst_QQuickLoader::loadedSignal depended on
sharing the engine with simultaneousSyncAsync
and that function being run before it.

After each test run call clearComponentCache()
as that is important for caching of the network tests.

The test server would send the item after 500ms.
Sometimes the CI would be faster or slower.
Instead of relying on timing, simply call a function
when the reply should be sent.

Change-Id: Ifb0447041197e1cba103570597a62a2510d31aab
Reviewed-by: Gunnar Sletta <gunnar.sletta@digia.com>
Reviewed-by: Friedemann Kleint <Friedemann.Kleint@digia.com>