Add tests for pullRequests.grouping syntax

[rtyley]

A colleague mentioned to me that some of our pullRequests.grouping were not pulling in all the artifacts we were expecting - while looking into this, I realised that I wasn't 100% clear on where wildcards could be used in pullRequests.grouping, so I wrote this test to exercise all the cases that were relevant to us, and see what worked - this may be tested elsewhere to some extent in the Scala Steward codebase, but I think it's helpful to drill into the specific functionality with these new tests on PullRequestUpdateFilter.

Note that the tests only assert the behaviour is what it is (meaning config parsing behaviour is tested to be consistent over time), not what it might 'ideally' be. By that, I mean that for myself, it would be ideal if the matching behaviour automatically included all sub-domains - so a filter of com.gu would match com.gu, com.gu.play-secret, com.gu.foo.bar etc, but not com.gumbo (because it's not a subdomain of com.gu) - but improving that behaviour would potentially affect how existing .scala-steward.conf files are interpreted - see "Matching group-&-sub-group-ids" in #3655.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 89.83%. Comparing base (083ed69) to head (942528d).
Report is 24 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #3654   +/-   ##
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  Coverage   89.83%   89.83%           
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  Files         174      174           
  Lines        5045     5045           
  Branches      495      495           
=======================================
  Hits         4532     4532           
  Misses        513      513           

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