Add tests for UsersManager (#1149)

Context

HomeDBManager lacks of direct tests, which makes hard to make rework or refactorations.
Proposed solution

Specifically here, I introduce tests which call exposed UsersManager methods directly and check their result.

Also:

    I removed updateUserName which seems to me useless (updateUser does the same work)
    Taking a look at the getUserByLogin methods, it appears that Typescirpt infers it returns a Promise<User|null> while in no case it may resolve a nullish value, therefore I have forced to return a Promise<User> and have changed the call sites to reflect the change.

Related issues

I make this change for then working on #870

stubs/app/server/server.ts

@@ -79,10 +79,6 @@ async function setupDb() {
       }
       const profile = {email, name: email};
       const user = await db.getUserByLogin(email, {profile});
-      if (!user) {
-        // This should not happen.
-        throw new Error('failed to create GRIST_DEFAULT_EMAIL user');
-      }
       db.unwrapQueryResult(await db.addOrg(user, {
         name: org,
         domain: org,