Add tests for focus()/blur() on MathML elements.

[fred-wang]

whatwg/html#12416

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[lukewarlow]

Does this need anything special for a elements (I don't think so right)? Also I seem to recall a test that exists already that chrome fails. We should make sure we're not duplicating tests here.

[fred-wang]

Does this need anything special for a elements (I don't think so right)? Also I seem to recall a test that exists already that chrome fails. We should make sure we're not duplicating tests here.

I think this is the generic test for focus()/blur() where we explicitly set tabindex=1, so it works with a too (in chromium too).

Indeed, there are other tests that relies on focus() but there more focused on a/tabindex, so this seems different.