make: clean tests better

make: clean tests better

1. 'make clean' in the root dir now also invokes 'make clean' in the
tests subdir so that it cleans up better recursively. The Makefile.am
does not list 'tests' as a normal subdir to avoid building that
directory for a normal make invoke.

2. 'make clean' in the tests/libtest and tests/unit directories now
*explicitly* remove the unity build executables even if this is not a
unit build. This, because those files may be leftovers from previous
builds and such leftovers can otherwise linger around and since
'runtests.pl' dynamically acts differently based on the mere *presence*
of those files, they can keep you fooled for a while until you
realize...

Closes #16986