lastIndexOf will convert the `needle` to unsigned char, which is then
'upcast' to int, before we search. But the string itself was searched
using signed char, meaning any values with the signed bit set would
mismatch.
Add tests for indexOf and lastIndexOf.
Amends 4c12ae7e67
Fixes: QTBUG-128199
Change-Id: I0ce7d7d9741f21650ef6f0f012a94e00d84a0f02
Reviewed-by: Ahmad Samir <a.samirh78@gmail.com>
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>
(cherry picked from commit 7b1f3bdc503ea7aceacc9fa8d388d843f1d7b131)
Reviewed-by: Qt Cherry-pick Bot <cherrypick_bot@qt-project.org>
5caa31cb21