Reimplement QML test cases of tst_loadUrl auto test

Get rid of some potential flaky issues:
- Fix typo in redirect.html
- Do not check URL before wait the result of the load. As a corner case
the load might be finished earlier than the comparison.
- Get rid of unnecessary signal handlers for unrelated test cases. The
superfluous operations might have enough overhead to cause a comparison
to be timed out.
- Separate data URL test case and blacklist it due a to known issue
(QTBUG-56661)

Task-number: QTBUG-56180
Task-number: QTBUG-56661
Change-Id: I76cec4e761be646c2f60588a34bae806cf558b3a
Reviewed-by: Kai Koehne <kai.koehne@qt.io>