Clean up notebook test utils

[fcollonval]

References

Follow up to #5000

Code changes

Export a notebook widget factory helper and mock context creator in NBTestUtils and fully use the utils in testutils from the notebook tests to ensure full capability for other extensions.

User-facing changes

Improved testing utilities for extension authors.

Backwards-incompatible changes

None

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[blink1073]

Thank you!

[blink1073]

@meeseeksdev please backport to 3.2.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 3.2.x
$ git pull

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

$ git cherry-pick -m1 f3a305fdbbdc36734f952915ded00dbc4b14fae7

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

$ git commit -am 'Backport PR #11021: Clean up notebook test utils'

4. Push to a named branch :

git push YOURFORK 3.2.x:auto-backport-of-pr-11021-on-3.2.x

5. Create a PR against branch 3.2.x, I would have named this PR:

"Backport PR #11021 on branch 3.2.x (Clean up notebook test utils)"

And apply the correct labels and milestones.

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

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

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