Fix deadlock in gdb-client tests

Using a listen queue of length 0 caused a deadlock on my machine in the
gdb-client tests while attempting to establish the loopback socket
connection.

I am not sure if this is down to a different python or kernel version,
but in either case, having queue of length zero sounds like a bad idea,
so I'm bumping that to one (which also fixes the deadlock).

llvm-svn: 341096