Add test for setting a non-agg Matplotlib backend

[ayshih]

This PR adds a regression test for #1102 via an example within tinybuild in the test suite. If the Matplotlib backend is reset to agg between code blocks, the example will raise an AssertionError during the build of tinybuild.

[larsoner]

I think you need to adjust the number of examples expected in tinybuild / sphinx_gallery/tests/test_full.py

[ayshih]

Oops, number now updated

[larsoner]

So far so good! Based on https://output.circle-artifacts.com/output/job/0dc682b1-1c23-4719-81d6-6e26cec1a970/artifacts/0/tiny_html/auto_examples/plot_matplotlib_backend.html it seems like a good test would be to check in test_full.py something like assert html_output.count('Matplotlib backend is svg') == 2, no?

[ayshih]

Sure, I can check the example print output in a unit test, but I should presumably then remove the assert statements in the example itself. If there is a regression, those assert statements would cause to the build to fail before the unit test even has a chance to run.

[larsoner]

Ahh you're right, somehow I missed the assert statements. Good enough, thanks @ayshih !

[ayshih]

Okay, great!