Fix flakiness in MediaSourceEngine unit tests
Fix flakiness in MediaSourceEngine unit tests

These tests relied on the status of whether or not TextEngine was
constructed, but never reset the spy between tests.  This fixes the
tests so that all spies are done beforeEach/afterEach instead of
beforeAll/afterAll.

Change-Id: I97db5bf37bbee7c8b60d75ef2db1b549159a2ec8

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