Add CI with more tests for OAR

It would be good to add a CI setup for OAR similarly to what is done for other schedulers, like Slurm and SGE. See https://github.com/dask/dask-jobqueue/tree/main/ci for more details.

Ideally a single docker image with both scheduler and workers would be simpler, but otherwise maybe https://github.com/oar-team/oar-docker could be useful.

Once this is done it would be great to add a few tests that need a real cluster (i.e. launch jobs) in `dask_jobqueue/tests/test_oar.py`. Right now all the tests actually check the job script but do not launch any jobs.