ESO tests: add more

This is initial work to add some more remote and local tests of the ESO query tools.  It has already revealed some things about the docs I didn't realize, in particular that you have to be logged in to download even public data.

@jwoillez: Are the hashes repeatable enough that we should always be able to use the Cache trick to make a request that is normally `@remote_data` behave as local data if we include the data in the repository?  i.e., will a normal request for, say, `Sgr A*` and `AMBER` make any attempt to connect to remote resources if it finds what it's looking for locally?  And, does the hashing system in the caching system apply to the submitted query only?