gitlab-ci: add tests for MIPS

Use the libatomic1:mips package only in the Debian Stretch Docker
image for MIPS and add Gitlab CI step to run tests on MIPS.

Signed-off-by: Guillaume Tucker <guillaume.tucker@collabora.com>
Fixes: 439a9f5d ("gitlab-ci: add build for MIPS")
Reviewed-by: Simon Ser <simon.ser@intel.com>