tests/guest-debug: add a simple test runner

The test runners job is to start QEMU with guest debug enabled and
then spawn a gdb process running a test script that exercises the
functionality it wants to test.

Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Tested-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Message-Id: <20200316172155.971-23-alex.bennee@linaro.org>