Improve the behavior of `valid_sql_file_migration`
Improve the behavior of `valid_sql_file_migration`

There were many cases where we would try to run a migration from a
directory that doesn't have the proper structure. This adds tests and
changes the behavior to ensure everything is set up properly.

What I really want to do is compare all the file names to `["up.sql",
"down.sql"]`, but it seems to be more difficult to do that than what we
have now.

diesel-rs/diesel,SoftwareWarlock/diesel,tohou/diesel,mcasper/diesel,tohou/diesel,diesel-rs/diesel,cascalheira/diesel,sgrif/diesel,mcasper/diesel,sgrif/diesel,SoftwareWarlock/diesel,cascalheira/diesel