Add tst_qwebenginescript loadEvents

Replace injectionPoint test with a timeout-less version that uses

- both a single-frame and a multi-frame page
- both main and isolated worlds
- cross-process navigation
- window.open
- profile-wide scripts

Task-number: QTBUG-66338
Change-Id: Ica4acb8ada4acc38aa5e1ca00e7512a2e69b785f
Reviewed-by: Michael Brüning <michael.bruning@qt.io>