Make shared examples and DummyAbility require-able from outside of core

[mamhoff]

Summary

This extracts all the work from #5634 that is essentially independent of extracting the promotion subsystem into its own gem, and simply allows requiring some of the shared examples and dummy classes we use in tests for other gems.

I've tried to do it in such a way that shared examples are only loaded where they're needed.

Checklist

Check out our PR guidelines for more details.

The following are mandatory for all PRs:

• I have written a thorough PR description.
• I have kept my commits small and atomic.
• I have used clear, explanatory commit messages.

The following are not always needed:

• 📖 I have updated the README to account for my changes.
• 📑 I have documented new code with YARD.
• 🛣️ I have opened a PR to update the guides.
• ✅ I have added automated tests to cover my changes.
• 📸 I have attached screenshots to demo visual changes.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (4b46992) 88.56% compared to head (8823049) 88.60%.
Report is 2 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #5640      +/-   ##
==========================================
+ Coverage   88.56%   88.60%   +0.04%     
==========================================
  Files         685      689       +4     
  Lines       16408    16473      +65     
==========================================
+ Hits        14531    14596      +65     
  Misses       1877     1877              

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

[kennyadsl]

👍 makes sense, thanks!

[tvdeyen]

Nice