fix and test annotations with mathjax

[alexcjohnson]

fixes #1787 - the bug came in when we simplified the class structure of annotation elements.
@etpinard another quick fix - lets hope the test image works consistently on CI. If it does, I'll probably add some more mathjax image tests (for elements other than annotations) when I tweak drawing.bBox for #1782

[etpinard]

Hmm. Looks like CI doesn't like MathJax intermittently.

[alexcjohnson]

Hmm. Looks like CI doesn't like MathJax intermittently.

Annoying. I guess this is probably the same sporadic delay we saw in https://github.com/plotly/streambed/issues/9508

I guess what I'll do is take MathJax out of the annotations mock, and make one MathJax mock that we skip in CI for now but tries to cover everywhere we support MathJax at present.

[etpinard]

I guess what I'll do is take MathJax out of the annotations mock, and make one MathJax mock that we skip in CI for now but tries to cover everywhere we support MathJax at present.

Very good idea, until we have more time to investigate plotly/streambed#9508

To do so, just add one expression to this filter function.

[alexcjohnson]

Actually I made a mistake in 033bee3 - fixed in b0d1af2 - that means it's likely MathJax will work fine on CI after all. But I found a bunch of other issues when developing a more complete MathJax test mock so I'm going to defer that to a separate PR if you don't mind.

[etpinard]

so I'm going to defer that to a separate PR if you don't mind.

Sounds good 👍

[etpinard]

💃 💃 💃