actions: coverage

[oliver-sanders]

closes: #3598
follow on from: #3899

Reinstates code coverage for Cylc.

It does:

• Capture all cylc commands.
• Write the coverage files in your Cylc working directory.
• Rsync coverage files back for code coverage on GH actions.
• Plug some minor coverage holes that were quick to write tests for.

It does not:

• Capture coverage for direct access to the cylc library from Python (i.e. not via the cylc CLI).
  • For this you can use standard tools directly (coverage, pytest --cov, etc).
• Capture anything after the daemonisation (i.e. when --no-detach is not set).
  • Can be done in the future though there is little to gain from doing so.
• Rsync coverage files back for code coverage locally.
  • Only does this on GH actions.

For assurance that a cylc command is being captured and covered set CYLC_COVERAGE=2, you will get a .coverage_commands_captured file which lists each command for which coverage was successfully recorded.

Initial results suggest we have ~89.5% coverage with obvious holes in the coverage around remote functionality, for now...

...Remote results in, 91.22%, not bad.

Requirements check-list

• I have read CONTRIBUTING.md and added my name as a Code Contributor.
• Contains logically grouped changes (else tidy your branch by rebase).
• Does not contain off-topic changes (use other PRs for other changes).
• Does not need tests (it is tests).
• No change log entry required (invisible to users).
• No documentation update required (documented in the code).
• No dependency changes.

[oliver-sanders]

Deconflicted

[MetRonnie]

tests/unit/main_loop/auto_restart.py → tests/unit/main_loop/test_auto_restart has lost its extension. Other suggestions optional

[kinow]

👏 great workaround !