The test was trying to synchronize a background decoding thread by
waiting for pending decode calls. However, the background thread needs
to fully queue the newly available output buffer before we can stop
waiting to ensure it's actually fully predictable. So we change the
pending wait to wait until the input buffer is cleared, which only
happens after the decoder is definitely done with it.
Also properly clean-up decoder (including shutting down the background
thread).
PiperOrigin-RevId: 316870659
