Tezt: JUnit reports

Context

See #1207 (closed).

This MR adds a --junit <FILENAME> command-line option to Tezt, to produce JUnit XML files that the CI can read. Then it modifies the CI to use this option.

This is based on !2679 (merged). This is not necessary but there would be significant conflict between the 2 MRs otherwise.

Manually testing the MR

dune exec tezt/tests/main.exe -- --junit tezt-junit.xml
cat tezt-junit.xml

You should probably restrict to a subset of the tests, e.g. with --file basic.ml, and modify a test so that it fails (and use --keep-going).

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, the Development Version section of CHANGES.md for everything else).
• Select suitable reviewers using the Reviewers field below.