Let CI run Python unit tests

[mkcor]

Closes #462.

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Description of changes

I was quite tempted to add a Python command right next to python unit_test_data_setup.py. Instead, should we separate Python unit tests and JS unit tests into two different steps?
Oops, that step of running Python unit tests definitely can't be part of a node job! I have rebased.

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[shammamah-zz]

should we separate Python unit tests and JS unit tests into two different steps?

I don't think so. The unit tests are meant to test components individually. Some of those components are written in Python instead of JS, but that doesn't mean they're two different types of tests.

[mkcor]

should we separate Python unit tests and JS unit tests into two different steps?

I don't think so. The unit tests are meant to test components individually. Some of those components are written in Python instead of JS, but that doesn't mean they're two different types of tests.

Oops, I have rebased and edited the description (see above) before seeing your comment... I agree they are both unit tests, but since the CI config is structured into node vs Python jobs, jobs being two levels above steps... it feels wrong not to have this pytest command under the Python jobs (i.e., 'where it belongs').

[Marc-Andre-Rivet]

💃 LGTM. One tiny bit more sanity.

I have no preference as to whether the Py unit tests and the JS unit tests should be executed together. Dividing JS / Py seems ok for now. Some projects in Dash combine them, some don't, based on the current structure.

[mkcor]

@shammamah by the way ;)

The unit tests are meant to test components individually.

This is not correct. Unit tests are meant to test code units (such as functions). The component level does not come into play; for convenience, we can use it in the file structure of course (e.g., file tests/unit/test_clustergram.py will gather tests for code units found in the Clustergram component). But the actual tests bear on code units.