CI: split the integration tests into a separate job

[DilumAluthge]

No description provided.

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (7358186) 57.89% compared to head (0901985) 57.89%.

Additional details and impacted files

@@           Coverage Diff           @@
##              dev    #1086   +/-   ##
=======================================
  Coverage   57.89%   57.89%           
=======================================
  Files           2        2           
  Lines          38       38           
=======================================
  Hits           22       22           
  Misses         16       16           

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[DilumAluthge]

@ablaom I think this approach makes it a bit easier to figure out (at a quick glance) whether or not the integration tests ran. Instead of having to look at the wall time of a job (and using the wall time to guess whether or not the integration tests ran), with this approach, if the integration tests don't run, they are marked as "Skipped" in GitHub, and they show up as a gray icon.

[ablaom]

This is a great idea!

If I understand this, in the dev -> master case we do run the non-integration tests twice (although in parallel jobs). I don't think this matters as these tests are pretty trivial. MLJ just collects other packages together, so there isn't much MLJ-specific testing.