test for the width and height attributes inside the image in the output

[mpacer]

This is in part a test improvement and in part meant to be an exposition on how to modify the tests in relation to #589 for @mscuthbert.

So, you want to check the content of the output, specifically we want to check whether there is a image element that has width and height attributes set. The '<img part is going to pick up the image src="[^"]*?" handles the base64 encoded data uri and ([^>]*?)>' grabs the remainder of the content until the tag is closed (which will include all of the attributes).

To run just this test it's easiest to run py.test nbconvert/exporters/tests/test_html.py from the top level directory of nbconvert.

To be able to inspect the state of the objects in the test, if you inject a python debugging trace (import pdb; pdb.set_trace()) that will inject you into a python interpreter at that point in the process and then you can play around with the actual content. So my first step was to insert that before the assert len(output)>0 and then got a handle on the converted content. I then dumped that into https://regex101.com/ (which I find to be a nice interface for testing regular expressions) until I got something that worked to capture the group that I wanted.

Then it's a matter of writing the assert statements that directly address what you about, in this case that there are width and height attributes that are present.