This patch makes the testsuite kill any running gdbserver before it
attempts to start a new one, so that there are never two running at
once (unless an error occurs, of course). This should make it easier
to reuse a single TCP port for multiple gdbservers, instead of eating
a huge range of ports. I also used it to run gdbserver over a serial
connection, where I can only start one at a time.
Tested m68k-uclinux and x86_64-linux and committed.