Increase timespan for WebViewFindText::test_findTextInterruptedByLoad

The timespans here are 5 seconds by default. These are not enough for
this test and make it fail often. We therefore increase the timespans
and un-blacklist the test.

Task-number: QTBUG-61887
Change-Id: I6acc2dee78fb4d589fd87f7557e0d48725b36f08
Reviewed-by: Kai Koehne <kai.koehne@qt.io>