Split functional tests into 4 parallel groups and add check-functional-parallel target (~3x faster CI runs).
Add branch-aware LCOV coverage reporting with new --enable-coverage option and lcov summary in CI pipeline.
Enable ccache via -C configure flag for faster recompilations.
Install lcov in all Docker images and use --depth 1 git clones + parallel make -j$(nproc) for quicker container builds.
Update CONTRIBUTING.md with instructions for parallel test groups and adding new ones.
All changes are tightly related CI/performance improvements developed in sequence. No external service uploads (e.g. Codecov skipped due to Gitea incompatibility).
## Summary
Fixes https://github.com/profanity-im/profanity/issues/1907
Update functional and unit test code to comply with the current cmocka test runner.
## Changes
- `UnitTest` struct to `CMUnitTest` struct
- `unit_test()` macro to `cmocka_unit_test(f)` macro
- `unit_test_setup_teardown()` macro to `cmocka_unit_test_setup_teardown` macro
- `run_tests()` macro to `cmocka_run_group_tests()` function
- Setup and teardown functions return `int` instead of `void`
## Testing
### Unit Tests
`make check`
### Functional Tests
I did not compile or run functional tests because they are *shelved* for now.
### Valgrind
I'm not entirely sure how to fun Valgrind in this case. I did not do fancy memory management, so it should be fine.
