mock: fix pts in SET_POSITION control

The PTS is a fraction of the length but should always be re-normalized by adding VLC_TICK_0 to be a valid timestamp.

test_input_decoder_mock decoder: Wait for the picture to be flushed from the vout
main input debug: control type=2
main input debug: ES_OUT_RESET_PCR called
main input error: Invalid PCR value in ES_OUT_SET_(GROUP_)PCR !