Add test files for high level interface

[Bultako]

This PR adds two simple files to test the use of $GAMMAPY_DATA in config variables of the high level interface, as it is described in gammapy/gammapy#3377

[adonath]

Thanks, @Bultako! Is adding the data here really needed? The exclusion mask is already here: https://github.com/gammapy/gammapy-data/blob/master/joint-crab/exclusion/exclusion_mask_crab.fits.gz

And the list of observations ids could be just generated in the test with a temporary file? I think adding a file for the two obs id's is not really worth it?

[Bultako]

@adonath

Ok for the exclusion mask, but I think the observation list file is needed. If we want to fully test the PR gammapy/gammapy#3377 I guess we need an observation list file in this GAMMAPY_DATA repo. Or I misunderstood gammapy/gammapy#3377 (review)

[adonath]

@Bultako Sorry my bad...in the case of obs_file I just meant adding a test at all. It doesn't have to use $GAMMAPY_DATA. I think we can be sure make_path() works, as we use it in many places, but from a quick look it seemed the that the obs_file option is not tested at all neither in test_analysis.py nor test_config.py.

[Bultako]

ok
I continue with the discussion in gammapy/gammapy#3377 and I will close this PR if no action is needed here to improve the test coverage.

[Bultako]

Closing since it is not needed in gammapy/gammapy#337