It'll help to make math equations "visible" for screen readers.
MS Office has a specific way to add some MathML code to struc tree leaf
and this patch handles it.
To avoid being able to introduce dependencies between tests this commit
makes sure that we close the document between tests so that we can't
accidentally rely on state set by a previous test.
The idea is to insert a span in the text layer with an aria-role set to img
and use the bounding box provided by the attribute field in the tag dict in
order to have non-null dimensions for the image to make it "visible".
