fix kj-test & wd-test test coverage reporting

[anonrig]

This pull-request ensures that Rust tests, wd_test and kj_test are generating proper test coverage (and with correct line coverages).

Current state of the test coverage on main branch can be accessed from https://app.codecov.io/gh/cloudflare/workerd

Improvements

Tests done to improve the speed of the test coverage runner:

• Tested different optimization levels for both deps and actual project.
• Enabled experimental_split_coverage_postprocessing which had the highest impact on reducing the coverage duration.
• Experiment with different v8-codegen-opt targets/options.
• Experiment with different optimization levels

Current Issues

Some ongoing issues with test coverage:

• It takes 2 hours to run on CI, hence it's better to run the coverage tests on main branch only.
• In order to remove, --experimental_outputs_writable flag, we need to land Fix COVERAGE_DIR permissions for split coverage post-processing bazelbuild/bazel#28311
• Rust tests are not properly generated after enabling experimental_split_coverage_postprocessing. In order to fix Rust test coverage, we need to land fix collect coverage script to work with recent changes bazelbuild/rules_rust#3812, and update our rules_rust.

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 69.67%. Comparing base (d19e973) to head (e13e91c).
⚠️ Report is 7 commits behind head on main.

Additional details and impacted files

@@             Coverage Diff             @@
##             main    #5881       +/-   ##
===========================================
+ Coverage   68.10%   69.67%    +1.56%     
===========================================
  Files          29      397      +368     
  Lines        2593   105999   +103406     
  Branches       15    17965    +17950     
===========================================
+ Hits         1766    73853    +72087     
- Misses        827    21351    +20524     
- Partials        0    10795    +10795     

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[anonrig]

Rebased and force pushed.

[anonrig]

Temporarily removed main branch requirement of coverage CI.

[anonrig]

Marking it as draft for now. While reverting the wd_test changes the coverage started failing again. Unfortunately, custom rule doesn't play well with coverage and we need sh_test in order to generate coverage reports correctly. I'll try to find a common ground.

[anonrig]

I've managed to get things going with the latest commit. I think this is ready to get merged. Let's for the CI to generate a coverage report first.

[fhanau]

LGTM otherwise, only suggestion is to check if any tests can be re-enabled now that coverage is a bit less slow.