test SignupUser displayname requirement

Adds displayname to existing assertions and new tests covering:
null/short/invalid-charset displayname, duplicate displayname,
findability by displayname. AuthenticateUser tests pick up the
seedUser displayname argument.

backend/tests/Unit/Auth/UseCases/AuthenticateUserTest.php

@@ -33,13 +33,16 @@ class AuthenticateUserTest extends TestCase
 
     private function seedUser(
         string $email,
+        string $displayName,
         string $password,
         bool $isAdmin,
     ): User {
         return $this->userRepo->create(new CreateUserDto(
             email: new EmailAddress($email),
+            displayName: $displayName,
             passwordHash: $this->hasher->hash($password),
             isAdmin: $isAdmin,
+            emailConfirmedAt: null,
         ));
     }
 
@@ -101,6 +104,7 @@ class AuthenticateUserTest extends TestCase
     {
         $this->seedUser(
             email: 'user@example.com',
+            displayName: 'user',
             password: 'correctpassword',
             isAdmin: false,
         );
@@ -116,6 +120,7 @@ class AuthenticateUserTest extends TestCase
     {
         $seeded = $this->seedUser(
             email: 'user@example.com',
+            displayName: 'user',
             password: 'correctpassword',
             isAdmin: false,
         );
@@ -140,6 +145,7 @@ class AuthenticateUserTest extends TestCase
     {
         $this->seedUser(
             email: 'admin@example.com',
+            displayName: 'admin',
             password: 'adminpassword',
             isAdmin: true,
         );