Fix flaky formgrader test

[jhamrick]

There is a flaky formgrader test in nbgrader/tests/nbextensions/test_formgrader.py called test_same_part_navigation which keeps failing with various related errors from Selenium, such as that the browsing context has been discarded or that it can't decode a response from Marionette, for example:

https://travis-ci.org/jupyter/nbgrader/jobs/438308641#L1083

I can't figure out why this is but maybe someone else can?

In #1026 I marked the test as flaky but when this bug is fixed that mark should be removed.

[jhamrick]

I should mention that part of the difficulty of debugging this is that I haven't been able to reproduce it locally---it only seems to fail on Travis 😞