CI: run matgrp tests as part of CI

[fingolfin]

... so we notice breakages sooner in the future.

matgrp is the only distributed GAP package (as far as I know)
which uses recog, and so we should be careful to not break it.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 88.80%. Comparing base (8d20f87) to head (71f0bba).
⚠️ Report is 23 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #486      +/-   ##
==========================================
+ Coverage   88.13%   88.80%   +0.67%     
==========================================
  Files          45       45              
  Lines       19023    19554     +531     
==========================================
+ Hits        16765    17365     +600     
+ Misses       2258     2189      -69     

see 22 files with indirect coverage changes

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