gsttestsrcbin: Avoid not-linked errors when switching tracks

gsttestsrcbin: Avoid not-linked errors when switching tracks

The previous implementation had a very high reproducibility race where
if after a track switch, the ex-active track pad completed a buffer
chain (now returning not-linked) the flow combiner had all their pads in
non-linked state, propagating it as an error and stopping the pipeline.

By resetting the flow combiner in response to RECONFIGURE events that
race is made impossible.