Modified gdb-remote tests to run with automatically-chosen ports.

Now that llgs supports communicating the 0-port choose-a-port
mechanism and can communicate that back to a caller via the
--named-pipe option (at parity with debugserver), we use this
mechanism to always start llgs and debugserver gdb-remote
protocol tests without needing to use some port arbitration
mechanism.  This eliminates some potential intermittent failures vs. the
previous random port and collision-avoidance strategy used.