Fix namespace of test classes (#3361)

Spotted with composer dump-autoload --strict-psr --optimize

Author: GromNaN

tests/Casts/EncryptionTest.php

@@ -2,7 +2,7 @@
 
 declare(strict_types=1);
 
-namespace Casts;
+namespace MongoDB\Laravel\Tests\Casts;
 
 use Illuminate\Database\Eloquent\Casts\Json;
 use Illuminate\Encryption\Encrypter;

tests/DateTimeImmutableTest.php

@@ -2,12 +2,11 @@
 
 declare(strict_types=1);
 
-namespace MongoDB\Laravel\Tests\Eloquent;
+namespace MongoDB\Laravel\Tests;
 
 use Carbon\CarbonImmutable;
 use Illuminate\Support\Facades\Date;
 use MongoDB\Laravel\Tests\Models\Anniversary;
-use MongoDB\Laravel\Tests\TestCase;
 
 use function assert;
 

tests/PropertyTest.php

@@ -2,10 +2,9 @@
 
 declare(strict_types=1);
 
-namespace MongoDB\Laravel\Tests\Eloquent;
+namespace MongoDB\Laravel\Tests;
 
 use MongoDB\Laravel\Tests\Models\HiddenAnimal;
-use MongoDB\Laravel\Tests\TestCase;
 
 use function assert;