Fail to set ADAT clock source on MacOS at sample rates >48000

[danielpieczko]

On MacOS, seen with xk_316_mc, 2AMi16o16xxxaax

1. xrun the DUT
2. set the stream format to the appropriate number of input and output channels for ADAT at sample rate >48000
3. start playing the ADAT input stream from the audio analyzer harness
4. use volcontrol to select the ADAT clock source
5. sometimes volcontrol fails with this error

   Found 2 clock sources
   Clock 0 = 690487296; XMOS Internal Clock
   Clock 1 = 724041728; XMOS ADAT Clock
   Current clock ID: 690487296
   Error 'ADAT' clock is not valid yet

[danielpieczko]

It turns out volcontrol is not checking the validity of the clock. It selects the ADAT clock source, sleeps for 1s and then gets the current clock source to check that the selection was applied. Also the implementation is different on Windows and MacOS, so that would explain why this hasn't been seen on Windows.