Fix visual tests, allow to disable termianal cursor blinking

[krassowski]

References

• Fixes Flaky Linux Tests / Linux (examples, 3.11) (pull_request) test (concurrency again?) #15510
• A large step towards Docs screenshots tests often fail (due to concurrency?) #15372: solves most flaky tests but does not address some underlying problems with galata, which are tackled in Some galata improvements #15375

Code changes

• add performance.throttleNetwork galata helper and use it to solve flakiness of completer test when trying to catch the loading indicator,
• move code for documentation tests which required serial mode from "general" to dedicated "overview" file, make them run in serial
• make customization docs tests run in serial
• mock the terminal tree command output for terminal snapshot
• switch to data directory in a few places to avoid test- directories form concurrent tests showing up in the snapshots

User-facing changes

We always supported cursorBlink in API but it was not exposed. This PR adds it to user-facing schema. We need it for tests, and for accessibility, but we should remove it from backport and only release in 4.1.

Backwards-incompatible changes

None

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[fcollonval]

Awesome 🚀
Thanks @krassowski

I have a minor suggestion. Otherwise it is good for me.

[jtpio]

Thank you @krassowski!!

[krassowski]

In one run these two failed, on re-run they passed. I think these will be addressed by #15375.

Notice:   2 failed
    [jupyterlab] › test/jupyterlab/debugger.test.ts:123:7 › Debugger Tests › Start debug session (Script) 
    [jupyterlab] › test/jupyterlab/notebook-run.test.ts:33:7 › Notebook Run › Run Notebook and capture cell outputs 

Good to merge?

[jtpio]

Thanks!

[krassowski]

@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 c26a0e7cced6eea64a7f0908a7cded8e4c8874fb

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

git commit -am 'Backport PR #15524: Fix visual tests, allow to disable termianal cursor blinking'

4. Push to a named branch:

git push YOURFORK 4.0.x:auto-backport-of-pr-15524-on-4.0.x

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

"Backport PR #15524 on branch 4.0.x (Fix visual tests, allow to disable termianal cursor blinking)"

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.