cbs: Add some read/write tests

Use the appropriate metadata filter for each codec - in the absence of any
options to modify the stream, the output bitstream should be identical to
the input (though the output file may differ in padding).

All tests use conformance bitstreams, the MPEG-2 streams are newly added
from the conformance test streams
<http://standards.iso.org/ittf/PubliclyAvailableStandards/ISO_IEC_13818-4_2004_Conformance_Testing/Video/>