qtestlib: -o filename should support placeholders for timestamp or PID replacement

Example use case: we execute a test 5 times as such:

tst_blah -o tst_blah_results.xml,xml

qtestlib then overwrites the output filename and we are only left with the last one. We should be able to inject the PID in the filename to make it unique, like this:

tst_blah -o tst_blah_results.%P.xml,xml