tests/acceptance: pick a random gdb port for reverse debugging

tests/acceptance: pick a random gdb port for reverse debugging

Currently the test randomly fails if you are using a shared machine
due to contention on the well known port 1234. We can ameliorate this
a bit by picking a random non-ephemeral port although it doesn't
totally avoid the problem. While we could use a totally unique socket
address for debugging it is fiddly to probe for gdb support. While gdb
socket debugging is not yet ubiquitous this a sub-optimal but workable
option.

Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Reviewed-by: Thomas Huth <thuth@redhat.com>
Reviewed-by: Pavel Dovgalyuk <pavel.dovgalyuk@ispras.ru>
Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Message-Id: <20201021163136.27324-7-alex.bennee@linaro.org>
Signed-off-by: Alex Bennée <alex.bennee@linaro.org>