tst_QProcess: Use a QTemporaryDir for files to be created.

Ensure the test works in a working directory with read-only
permission and that the file names are unique.
The test can then be executed repeatedly by COIN even in case
left-over hanging process helpers still lock the files. Also
disambiguate the "data" files used by various tests.

Task-number: QTBUG-47370
Change-Id: I3b9c7b70828da78f400196fcbba27bc61ea4538f
Reviewed-by: Jędrzej Nowacki <jedrzej.nowacki@theqtcompany.com>