Test tooling: Use stricter regex for lines in expected_results.txt files, provide readable errors for format errors

[vmichal]

When parsing expected_results.txt for some test suite, the python script was not ready to handle the following issues:

• Missing result code (only file name present) yielded generic "AttributeError: 'NoneType' object has no attribute 'group'" when attempting to access regex match groups.
• Typo in result code yielded "AttributeError: module 'stl.test.tests' has no attribute", which is arguably better, but still could be better.

Both issues are quite easy to trigger when experimenting with a larger number of tests, so user-friendly error messages seem like a nice touch.

[vmichal]

BTW I am more than open to alternative wording of error messages. I experimented with a few alternatives and this one I disliked the least.