CI: hotfix failing answer tests

[neutrinoceros]

PR Summary

fix #3256 (?)
For now I'm just commenting out two tests functions that I strongly suspect are the ones generating the failing answers.
If CI validates my suspicions I'll be able to start digging into it for real.

update for posterity:
since I've rebased and force pushed a different fix, there is no trace left of the tests functions I was referring to, so here they are:

• yt/visualization/tests/test_line_plots.test_line_plot
• yt/visualization/tests/test_line_plots.test_multi_line_plot
  eg the only tests that use the LinePlot class (that I'm aware of)

[neutrinoceros]

Looks like I was right, now let's bisect it down.

[neutrinoceros]

Good news, the issue seem to lie in an annotation method, which narrows it down much better than if it was in yt.LinePlot.init

[neutrinoceros]

After a couple iterations I established that calling any of the following methods causes the ticks to be inverted:

• LinePlot.annotate_legend
• LinePlot.set_x_unit
• LinePlot.set_unit
• LinePlot.annotate_title

The common denominator is that all these use the @invalidate_plot decorator, which forces self._plot_valid = False.
Then I need to understand when that invalidation is going to affect the result and how, and it seems to quickly get a lot messier, as the dataflow goes back and forth between several classes and _setup_plot methods. It would be extremely valuable to narrow it down to the part where we force inward ticks, but atm I can't find it.

update: it's very likely this line
https://github.com/neutrinoceros/yt/blob/f20e3e7281dc893110373e165d147526d6f781e9/yt/visualization/base_plot_types.py#L97

[neutrinoceros]

I was able to confirm that the regression is indeed upstream (reported as matplotlib/matplotlib#20219)
So I will patch this by updating our requirements (forbid 3.4.2)