Testing: Test identifier functions

[hamogu]

Most test in https://github.com/astropy/specutils/blob/master/specutils/tests/test_loaders.py actively set the loader, e.g.:

spec = Spectrum1D.read(tmp_file.name, format="SDSS-III/IV spec")

All these tests should be parameterized to be run twice: With and without the format keyword to make sure that the identifier functions work. (This is very analogous to the astropy.io.ascii tests and experience there shows really both tests are needed.)