Test: hopefully more robust PSYNC test.

This is supposed to fix issue #1417, but we'll know if this is enough
only after a couple of runs of the CI test without false positives.
Remove infinite loop from PSYNC test.

Added for debugging and forgot there.
CLIENT KILL: don't kill the master as a normal client.

Technically the problem is due to the client type API that does not
return a special value for the master, however fixing it locally in the
CLIENT KILL command is better currently because otherwise we would
introduce a new output buffer limit class as a side effect.