QString::insert(qsizetype, QChar) can insert at negative positions,
then counting from the end of the string. Coverage analysis revealed we
do not have a unit test for this. This patch adds a unit test.
Pick-to: 6.0
Change-Id: I8d41b38df964c07fe2d2e7be444f8236c9e19b5d
Reviewed-by: Sona Kurazyan <sona.kurazyan@qt.io>