This patch introduces some test improvements to check the calls of
different methods on an empty default-constructed string.
Apart from that, some other tests are added to extend code coverage.
As a drive-by:
* fix int -> qsizetype in the test data
* fix int -> enum in the test data
Task-number: QTBUG-91736
Pick-to: 6.2 6.1
Change-Id: I159473b7f5dcbea1bdaf2966979e066296351208
Reviewed-by: Edward Welbourne <edward.welbourne@qt.io>
Reviewed-by: Andrei Golubev <andrei.golubev@qt.io>
8ac686e9f4