Fix test framework to detect proper server PID

Previously the PID format was:
`[PID] Timestamp`

But it recently changed to:
`PID:X Timestamp`

The tcl testing framework was grabbing the PID from `\[\d+\]`, but
that's not valid anymore.

Now we grab the pid from `"PID: <PID>"` in the part of Redis startup
output to the right of the ASCII logo.