Add tests against GAP stable-4.14 and stable-4.15

[wilfwilson]

Since the GAP stable-4.15 branch has been created.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 62.09%. Comparing base (6b260d6) to head (e26c2bf).
⚠️ Report is 2 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master      #48   +/-   ##
=======================================
  Coverage   62.09%   62.09%           
=======================================
  Files           3        3           
  Lines         153      153           
=======================================
  Hits           95       95           
  Misses         58       58           

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