Enable commented tests on Travis CI

[pllim]

I saw these commented in .travis.yml with a note that something needs to be done, so I am just opening an issue here so they don't get forgotten:

• [ ] Enable tests for different toolkits (Qt, GTK, etc).
• [x] Enable -w option for build_sphinx test. (done in #548)
• [ ] Enable coverage test.

[pllim]

Note from Scipy 2018: To enable tests for different toolkits, we can compare PNG screen captures. For example, see compare_images() in https://github.com/matplotlib/matplotlib/blob/master/lib/matplotlib/testing/compare.py#L361 . The limitation is that this limits the testable region to image display region; i.e., you cannot test GUI elements (e.g., did hitting "Cancel" work as expected?). However, something is better than nothing...