This was more to fix a problem the test were having, as well as being a little bit more functionally correct.  The `def migration-files` was only called once, when the namespace was loaded.  In the tests, the `with-redefs` call is happening after the `clj-sql-up.migrate` namspace is loaded, so we were unable to effect which directory was loaded.  By querying for the files when we need them, this became quite testable.

As for the additional case, it was being used in an earlier commit, but I removed the need for it.  I can take it out -> though, I'll be replacing it with a `let` so the migration files are guaranteed to be the same.