tests/docker: fall back more gracefully when pull fails

I only spotted this in the small window between my testing with my
registry while waiting for the gitlab PR to go in. As we pre-pull the
registry image we know if that fails there isn't any point attempting
to use the cache. Fall back to the way we used to do it at that point.

Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Message-Id: <20200709141327.14631-12-alex.bennee@linaro.org>