Tests/Coverage: Store coverage trace and render coverage html in tree form

Context

Two small improvements to coverage:

Store traces as artifacts

The coverage traces are computed in the CI but not stored. Only the HTML report is stored. Often, it is convenient to store the trace for debugging.

Render coverage HTML report as a tree

Pass the option --tree to bisect-ppx-report, activating the feature of https://github.com/aantron/bisect_ppx/pull/396

Manually testing the MR

Check out the artfacts of unified_coverage: https://gitlab.com/tezos/tezos/-/jobs/2573375171/artifacts/browse/.

In the _coverage_output folder, you will now find the traces (.coverage files).

In the _coverage_report folder, you will find the HTML report as usual, but now in tree form.

Checklist

• Document the interface of any function added or modified (see the coding guidelines)
• Document any change to the user interface, including configuration parameters (see node configuration)
• Provide automatic testing (see the testing guide).
• For new features and bug fixes, add an item in the appropriate changelog (docs/protocols/alpha.rst for the protocol and the environment, CHANGES.rst at the root of the repository for everything else).
• Select suitable reviewers using the Reviewers field below.
• Select as Assignee the next person who should take action on that MR