Refactor tests for XNAT Central and NITRC-IR

[bpoldrack]

As mentioned in PR #121, refactor the tests in tests/test_xnatcentral.py and tests/test_nitrcir.py in order to have generic tests, that would loop over definitions of a targeted XNAT instance and project/subject/etc. to test against. This could involve authenticated targets later on as well (whenever we find a way to test that from within CI).

The tests in both files are already as good as identical and rely just on differently specifiec constants at the top of their respective file. Should like become a dict or some other structure rather than a collection of constants.