tests/acceptance: disable machine_rx_gdbsim on GitLab

While I can get the ssh test to fail on my test setup this seems a lot
more stable except when on GitLab. Hopefully we can re-enable both
once the serial timing patches have been added.

Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Reviewed-by: Willian Rampazzo <willianr@redhat.com>
Reviewed-by: Cleber Rosa <crosa@redhat.com>
Message-Id: <20201007160038.26953-23-alex.bennee@linaro.org>