Fix overrides.json not working for shortcuts

[krassowski]

References

Fixes #15713

Code changes

• add test ensuring that the default settings (such as from overrides.json) are respected - it should fail in the first second commit (it seems this test group was not run on CI, second commit fixes it)
• fix the overrides getting lost

User-facing changes

Shortcuts modifications defined in overrides.json now work.

Backwards-incompatible changes

None

[jupyterlab-probot]

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[krassowski]

This test is now executed correctly on CI:

[jtpio]

Thanks!

[jtpio]

@meeseeksdev please backport to 4.0.x

[lumberbot-app]

Owee, I'm MrMeeseeks, Look at me.

There seem to be a conflict, please backport manually. Here are approximate instructions:

1. Checkout backport branch and update it.

git checkout 4.0.x
git pull

2. Cherry pick the first parent branch of the this PR on top of the older branch:

git cherry-pick -x -m1 63651b410191e61fea57068818c250727dd28e8b

3. You will likely have some merge/cherry-pick conflict here, fix them and commit:

git commit -am 'Backport PR #15716: Fix `overrides.json` not working for shortcuts'

4. Push to a named branch:

git push YOURFORK 4.0.x:auto-backport-of-pr-15716-on-4.0.x

5. Create a PR against branch 4.0.x, I would have named this PR:

"Backport PR #15716 on branch 4.0.x (Fix overrides.json not working for shortcuts)"

And apply the correct labels and milestones.

Congratulations — you did some good work! Hopefully your backport PR will be tested by the continuous integration and merged soon!

Remember to remove the Still Needs Manual Backport label once the PR gets merged.

If these instructions are inaccurate, feel free to suggest an improvement.