Bug in qt5 testcase.prf

[nullmedium]

A patch that is applied to qt5 testcase.prf breaks the "check" target.

The use of $$system_path for $$TESTRUN_CWD creates a Windows style path. If the target
is executed within the msys shell, the windows style path doesn't work when changing to the testcase directory.

[ossilator]

yes, system_path() is bogus; it must be shell_path() here.

see also https://codereview.qt-project.org/217175 ; i'd expect 5.9 HEAD to work out of the box with msys now.

[mati865]

Mentioned patch https://github.com/Alexpux/MINGW-packages/blob/5085b03c4281c9966817a8a0746966486b55b912/mingw-w64-qt5/0028-qt-5.8.0-Revert-untangle-use-of-system-vs.-shell-path-list-se.patch#L67

P.S.
This issue is for MINGW-packages repo.