tests/functional: Add a test for the arm microbit machine

tests/functional: Add a test for the arm microbit machine

We don't have any functional tests for this machine yet, thus let's
add a test with a MicroPython binary that is available online
(thanks to Joel Stanley for providing it, see:
 https://www.mail-archive.com/qemu-devel@nongnu.org/msg606064.html ).

Signed-off-by: Thomas Huth <thuth@redhat.com>
Reviewed-by: Alex Bennée <alex.bennee@linaro.org>
Message-id: 20250124101709.1591761-1-thuth@redhat.com
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>