Add tests to exercise seeking in cueless WebMs. These tests reproduce a subset
of the test_seek tests, namely the first 3. These tests have been added
separately from the test_seek ones to avoid additional logic required to handle
cueless WebMs in such test. Once random seeking for cueless WebMs is
implemented then these cases can be easily rolled into the existing test_seek
cases by moving the cueless WebM into the appropriate test array in the
manifest -- after which these tests can be removed.
MozReview-Commit-ID: DNwiSmMmALQ
