This is work torward the ability to run the testbench in parallel.
Some small issues in tests have also been detected, which were not
previously seen. They have been fixed. We did not do separate
commits for this as it would clutter the commit log with not
really relevant info.
Also move some cleanup to "make clean" target
To support parallel testbench runs, we need to have dynamic work files.
So deleting work files on each test does not work, especially as we can
no longer assume they are "left-overs" from a failed test - they may
actually (and quite likely) belong to tests that run in parallel.
So the proper solution is to do via "make clean".
closes https://github.com/rsyslog/rsyslog/pull/2916
