Test suite: Clean Apache logs, cache, and test outputs on make mostlyclean
Test suite: Clean Apache logs, cache, and test outputs on make mostlyclean

This provides a good way to clean out the test directory without forcing
a regeneration of test keys and certificates, saving time and entropy.
Also use proper Automake variables when calling TestMakefile.

airtower-luna/mod_gnutls,airtower-luna/mod_gnutls,airtower-luna/mod_gnutls