Add spPlate test

[rosteen]

Previously, this loader/identifier wasn't included in the __all__ of the relevant file, which was fixed in #1097. We should add a test for this loader to make sure those continue to work properly.

[dhomeier]

Are the ones in test_sdss_spplate not doing this? Though the remote files are rather huge I think, not sure if they are run anywhere except perhaps in one of the crons.