runtc: cope with "late" samplerate announcement

runtc: cope with "late" samplerate announcement

The previous implementation expected a rather specific sequence of
SR_DF_HEADER with samplerate, then SR_DF_LOGIC. This worked with .sr
formatted session files. But does not work with other input formats.

Also process SR_DF_META packets. Pass any samplerate that we learn about
to decoders' .metadata() method. Defer decoders' .start() until the first
logic samples are seen. This improves robustness of the decoder tests.