test: Add more unit tests

Hello,

I've been reviewing the codebase and it looks like we could use some more unit tests in certain areas.
Here is a list of locations that existing unit tests don't cover:

`firebase.google.com/go/v4/db/db.go`

`firebase.google.com/go/v4/auth/user_mgt.go`




Would it be possible for me to contribute by adding these missing tests? Any guidance or pointers in this regard would be greatly appreciated.

Looking forward to your response.