Improve test diagnostics when expected_results.txt contains syntax errors

[vmichal]

When a formatting error is encountered while parsing expected_results.txt, report the line number as well.

The situation notably improved in #5948, but there is still a tiny improvement to be made.

[StephanTLavavej]

Thanks! 😻 I hulked out check_libcxx_paths.py to more intelligently complain about damaged lines (those whose syntax appears to be very malformed), unknown results (the common mistake of saying SKIP instead of SKIPPED), and nonexistent tests only if the syntax is otherwise valid. This is a vast improvement over the earlier behavior, thanks for prompting me to do this.

check_libcxx_paths.py doesn't give line numbers but I believe its output is easy enough to track down. This is a nice belt-and-suspenders approach.

[StephanTLavavej]

I'm mirroring this to the MSVC-internal repo. Please notify me if any further changes are pushed, otherwise no action is required.

[StephanTLavavej]

Thanks for making it easier to understand test harness failures! 🧠 💡 😸