currentTextChanged is emitted when the current index changes
and the current text doesn't.
This can be the case, if
- old and new index have identical text values
- an item is removed below the current index
[ChangeLog][Widgets][QComboBox] emit currentTextChanged only,
if currentText changes.
Add a corresponding test in tst_QComboBox::currentText().
Fixes: QTBUG-113717
Pick-to: 6.6
Change-Id: I847874f0792b29a2841e50bb82d06ad496fb02c3
Reviewed-by: Richard Moe Gustavsen <richard.gustavsen@qt.io>