…e strict formatting VOD files require.

- feat(stream selector): allow to explicitly set the input format, in case autodetection is faulty (as is the case with LIVE WebVTT).

The newer, more tolerant header check, allows to ingest webvtt without properly detecting header.
Even in such case, the format autodetection procedure later fails to detect webvtt, so the output for the unit test is still "no streams".
So the only needed change is the failure to parse to a successful parse, but mantaining the empty streams assert.

…g with a mention to the new "input_format" selector option.