Add extendable test framework

[sol1105]

The idea:

• in tests/_commons.pynew test datasets can be added
• in tests/test_cc6.pyone can specify if/which checks for this test dataset fail, and why
• All test datasets referenced in tests/test_cc6.py:TestCC6Checks.test_all_cc6_checks will be checked by each individual cc6 check and unexpected check-failures reported

This should allow to quickly add new test datasets to the CI checks.

(A few problems I discovered in the check methods have been addressed as well.)

[larsbuntemeyer]

Thx, i'll have a look asap!