Extend tests to explicitly check the behavior of empty
default-constructed containers.
Also add some missing tests for the existing methods
(mostly for QMultiHash) and correct some end()s to cend()s
in comparisons.
Task-number: QTBUG-91736
Pick-to: 6.2 6.1
Change-Id: Ic9e1b86ef67f6bca2751a65a8589b2f7e0ebb5ea
Reviewed-by: Andrei Golubev <andrei.golubev@qt.io>
Reviewed-by: Edward Welbourne <edward.welbourne@qt.io>