Remove the insignificant_tests CONFIG option in favor of a BLACKLIST
file. The tests blacklisted have been found using CI builds logs.
Change-Id: I1a963bdc24f7657731dc0374a8e2c3cbaa49f126
Task-number: QTBUG-22775
Reviewed-by: Morten Johan Sørvig <morten.sorvig@theqtcompany.com>