Refactor wait for HLS/DASH manifests in tests
Refactor wait for HLS/DASH manifests in tests

This cleans up and refactors the waiting system in
run_end_to_end_tests.py so that it is more readable and independent of
specific filenames.

Change-Id: I63e8652c57bd48fd77a015bc2ecc471e4176471a

shaka-project/shaka-streamer,shaka-project/shaka-streamer