The mxf_d10 muxer is very picky regarding the input it accepts:
The only video accepted is MPEG-2 with absolutely constant bitrate,
i.e. all packets need to have exactly the same size; and only a few
bitrates are accepted.

The sample file used did not abide by this: Writing the first packet
(a video packet) errors out and afterwards an audio packet from the
muxing queue has been written. That's all besides metadata (which this
test is about). The FFmpeg cli returned an error, but said error has
been ignored by the md5 test.

This commit changes the test to actually send a compliant stream to the
muxer, so that it does not error out; furthermore, the test is changed
to explicitly check the metadata instead of it only being implicitly
included in the md5 checksum. The compliant stream is created by our
encoder at runtime.

Finally, the test now also covers writing user-specified
product/company/version identification.
Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@gmail.com>