Add mediatestutils and CancelAllFuturesRule

Add new utils library for media tests.

Add a CancelAllFuturesRule which cancels all registered futures on test
completion (regardless of success).

Add unit tests.

Bug: 294636572
Bug: 288333346
Test: atest CancelAllFuturesRuleTest
Change-Id: If270a6b7683515157ef356c41ea53ed872bd85d8