[SDTEST-138] code coverage extension fixes and improvements

[anmarchenko]

What does this PR do? Applies the following changes and optimizations to per test code coverage:

• removes lines code coverage mode as it is not used and is not going to be used in the near future, so it is just a dead code right now
• caches last seen filename pointer so that if we cover sequential lines in the same file we skip prefix check and saving file name in the coverage hash table
• dd_cov_data->root and dd_cov_data->ignored_path are now copied (once) in malloc'd memory and stored as C-strings
• to avoid string copying, rb_profile_frames function is now used to store filename in resulting hash: it improves performance and solves UTF-8 filenames issue

How to test the change? Tested using test-environment integration test stand: https://github.com/DataDog/test-environment/pull/283

The following changes in performance overhead were recorded:

Rubocop From 109% to 86% => 27% improvement

Middleman From 29,7% to 28,6% => no change

Jekyll From 0,4% to 0,3% => no change

devdocs From 16,4% to 16,9% => no change

Overall result: it does not change anything for projects where code coverage overhead was low enough but provides meaningful improvement for the project where code coverage overhead was the largest.

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 98.85%. Comparing base (f58bb35) to head (b04133e).

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #171 +/- ## ========================================== - Coverage 98.86% 98.85% -0.01% ========================================== Files 228 228 Lines 10096 10082 -14 Branches 466 465 -1 ========================================== - Hits 9981 9967 -14 Misses 115 115 ```

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[anmarchenko]

@ivoanjo yes, the number for performance improvements is final and was measured with the current state of this PR. Indeed, the switch to rb_profile_frames caused more overhead than rb_sourcefile but it is safer and solves an issue with UTF-8 filenames, so I consider it being worth it.