Cleanup DOM after text displayer tests
Cleanup DOM after text displayer tests

These tests added a container to the DOM which was never cleaned up.
There is no impact to Shaka Player, and no performance impact to the
tests, but we should clean up after ourselves.

I found this by running the Karma/Jasmine tests in a foreground
browser tab at http://localhost:9876/debug.html .  After the tests
completed, I looked at the "Elements" panel in Chrome's dev tools.

Change-Id: Id81dddcb82b01eeb6445ddd701d2d3907c322a0d

tvoli/shaka-player,shaka-project/shaka-player,tvoli/shaka-player,tvoli/shaka-player,tvoli/shaka-player,shaka-project/shaka-player,shaka-project/shaka-player,shaka-project/shaka-player