Add skylib tests for compile_commands.bzl

[furtib]

Why:
We want fast and closely integrated tests. Skylib provides functions for Bazel-level unit testing.

What:

• Refactored compile_commands.bzl to be more easily testable. Notably:
  • Made check_source_files and compile_commands_json functions public.
  • Added a short description for these newly public functions.
  • Moved the fail() call out of check_source_files so it is properly testable.
• Added skylib unit tests for all functions that can be analyzed with them (basically all functions that don't use ctx or providers)
• Added skylib analysis tests for functions that can't be analyzed with unit tests. These tests cannot test one function individually; they can only assert on the final output of the aspect, using the cc_binary/library targets from the BUILD file.

Notes:

• I added a test for compilation flag duplication.

Addresses:
none

[furtib]

This PR is waaaay too big; will split into several pieces.
Starting with the expected to fail tests.

[furtib]

Superseded by: #284, and more to come.