plots: Expand flexible plots tests

[daavoo]

Yes, its true. I will fix that in follow up PR, where I intend to parameterized this test to handle one use case per test. That way we won't have to create this huge repo_with_plots.
This specific use case (list of columns) is tested in tests/unit/render/test_vega_converter.py::test_converter - its a unit test but should be enough for now.

Originally posted by @pared in #7477 (comment)

[pared]

Aim of this issue is to parametrize tests in tests/integration/plots/test_plots.py, so that we have single use case per parameter set, and not testing few use cases at once, as we do now. That will allow us to test every expected plots set up with a test that is much easier to comprehend.

[pared]

Need to test #8004 too

[pared]

Include test for #8159

[dberenbaum]

@daavoo What's still needed here?

[daavoo]

@daavoo What's still needed here?

No idea off the top of my mind. I would need to take a look. At this point, I think most behaviors we care about are somehow tested in here or dvc-render or studio. But revisiting the tests would not hurt

[dberenbaum]

Okay, feel free to close this one or take another look.