demux: mock: generate PCR with vlc_tick_wait

Using vlc_tick_sleep means that errors due to scheduling are added after each sleep, and that time between two sleeps is not accounted.

     SetPCR     SetPCR   SetPCR
     v          v        v
|----|   |------|   |----|
       ^      ^ Error not accounted
Time between SetPCR not accounted
-- Example of bad scheduling leading to increasing error.

Using vlc_tick_wait with deadline incremented by the PCR step means that systematic scheduling errors are not summed and only bias the call to SetPCR by adding a const delay. It's also more robust against delay between each SetPCR call.

     SetPCR    SetPCR    SetPCR
     v         v         v
|    |    |    |    |    |
|----|     |---|   |-----|
-- Example of bad scheduling without increasing error.

Since SetPCR calls are not affected by a constant systemic error on the time of each call (since the reference also has the error), there's no problem removing the delay delay / 1000 also.

       SetPCR    SetPCR    SetPCR
       v         v         v
|    |    |    |    |    |
|----|     |---|   |-----|
-- Example of constant error.