update example unit test to use RefreshDatabase trait

tests/Unit/ExampleTest.php

@@ -3,8 +3,7 @@
 namespace Tests\Unit;
 
 use Tests\TestCase;
-use Illuminate\Foundation\Testing\DatabaseMigrations;
-use Illuminate\Foundation\Testing\DatabaseTransactions;
+use Illuminate\Foundation\Testing\RefreshDatabase;
 
 class ExampleTest extends TestCase
 {