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.
Merge pull request #1775 from mattsta/fix-test-against-new-PID-format

Fix test framework to detect proper server PID