Make notebook tests less flaky

[danielhollas]

We now wait until the Appmode loading gif disappears. We've been using this strategy in my app and in QeApp for a while and it seems to work well.

[codecov[bot]]

Codecov Report

:exclamation: No coverage uploaded for pull request base (master@3f15948). Click here to learn what that means. Patch has no changes to coverable lines.

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Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #497 +/- ## ========================================= Coverage ? 79.63% ========================================= Files ? 27 Lines ? 3752 Branches ? 0 ========================================= Hits ? 2988 Misses ? 764 Partials ? 0 ``` | Flag | Coverage Δ | | |---|---|---| | python-3.10 | `79.63% <0.00%> (?)` | | | python-3.8 | `79.67% <0.00%> (?)` | | | python-3.9 | `79.67% <0.00%> (?)` | | Flags with carried forward coverage won't be shown. [Click here](https://docs.codecov.io/docs/carryforward-flags?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=aiidalab#carryforward-flags-in-the-pull-request-comment) to find out more.

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[danielhollas]

Ugh, looks like new Cython broke tests here as well: scikit-learn failed to build. I guess we should upgrade it.

[danielhollas]

@unkcpz this one is now ready.