Borrow a few login/currentUser tests from Snap's test suite

These test will exercise at least lookupByUserId backend function
which was previously uncovered.

The tests come from commit ad60dbf4 from
https://github.com/snapframework/snap.  Unfortunately that commit is
made by "Your Name" from example.com, so I don't know exactly who to
attribute this to. :)
Simplify user save error logic by removing it

The code wouldn't return an AuthError anyway, so just get rid of the
pattern match that'd never be run.