Split the code out of QDir::fromNativeSeparator into a separate
reusable function to remove the above-mentioned prefixes. Fixes
and unifies behavior if the prefix was given with slashes instead
of backslashes. Add a couple more test cases.
Fixes: QTBUG-93868
Pick-to: 5.15 6.0 6.1
Change-Id: Ibd94ae283e2fb113f9c2db97475fbc7d89522bbf
Reviewed-by: Edward Welbourne <edward.welbourne@qt.io>
Reviewed-by: Oliver Wolff <oliver.wolff@qt.io>
0564ebdb36