Add generic mlj interface tests

[ablaom]

Adding the tests is not a requirement for MLJ registrtation, but I recommend it. This adds maybe a minute to the test for each platform.

I have also dumped MLJ as a test dep in favour of StatisticalMeasures. Everything except the measures is already provided by MLJBase.

I notice that test/Manifest.toml is committed which seems unusual. I haven't done anything about that.

[pasq-cat]

@ablaom yeah the manifest is probably my mistake. i may have uploaded it and forgot. @pat-alt i think this can be merged , right?

[pasq-cat]

i will approve it. let's hope patrick doesn't cross the Alps to kick my butt xD

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 97.00%. Comparing base (5f66429) to head (7432047).
Report is 3 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #131   +/-   ##
=======================================
  Coverage   97.00%   97.00%           
=======================================
  Files          22       22           
  Lines         734      734           
=======================================
  Hits          712      712           
  Misses         22       22           

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[pasq-cat]

sure is better than what i wrote