Platform API: Use DatabaseTransactions instead of deleting all users

Cleaning up from tests with `User::query()->delete();` is a bad idea as it will delete all users rather than just the ones created in `CheckUserEmailExistTest`.

Use Laravel's `DatabaseTransactions` trait instead that wraps each test case in a database transaction and only reverts database modifications cause by the test.

Bug: T415636

Author: outdooracorn

tests/Commands/User/CheckUserEmailExistTest.php

@@ -4,14 +4,12 @@
 
 use App\Services\WikiUserEmailChecker;
 use App\User;
+use Illuminate\Foundation\Testing\DatabaseTransactions;
 use Mockery;
 use Tests\TestCase;
 
 class CheckUserEmailExistTest extends TestCase {
-    protected function tearDown(): void {
-        // delete all users
-        User::query()->delete();
-    }
+    use DatabaseTransactions;
 
     public function testItFindsEmailInApiUsersTable() {
         User::factory()->create([