split db tests into multiple namespaces (to be consistent with app code)

[rafd]

Our braid.server.db namespace was split into several namespaces (ex. braid.server.db.user, braid.server.db.group), but the corresponding tests namespace is still one giant file (braid.server.test.db).

Split braid.server.test.db into multiple namespaces (mirroring the the actual braid.server.db.* namespaces).