code cleanup + improvements for easyconfigs test suite

[Flamefire]

Major improvements to the test suite

• Remove a forgotten debug print
• Checkout full history on CI to avoid failure to get merge base
• Remove workaround to cleanup test TMPDIR in favor of class teardown method
• Process ECs on demand (preparation for next change, but also means for non-prs the ecs-in-pr-changed test is a no-op instead of parsing all ECs and never using them)
• Refactor the ECs-changed-in-PR test to run the subtests as separate test cases so failures in one of them don't abort the others
• Ignore cmdline args passed to the test suite, those are handled by unittest and should not be handled by easybuild. This allows to run subsets of tests like: python test/easyconfigs/easyconfigs.py -v -k test_pr to run all PR tests only
• Remove unused setting of EASYBUILD_DEPRECATED which, if in effect, would cause many failures due to deprecated ECs (using old versions) or even using LMod 6 (on CI) is already deprecated
• Honor setting of EASYBUILD_TMP_LOGDIR
• Allow filtering over all tests via: python test/easyconfigs/suite.py -k foo which includes the EC parsing tests. This removes the custom output of "ERROR: Not all tests were successful." but still has the output from unittest

[boegel]

lgtm