tests: ensure output mocking is disabled so we can actually catch the output (#1018)

Discovered via https://github.com/barryvdh/laravel-ide-helper/pull/1017#issuecomment-679318764

This only happens if you run an artisan command _before_ `runCommand`,
but since every of these tests performs migrations, using artisan,
the mocked output instance is left behind and also affected us.

The call added in `runCommand` explicitly unregisters
`\Illuminate\Console\OutputStyle` and thus also throwing away possibly
mocked versions.

tests/Console/ModelsCommand/MagicWhere/Test.php

@@ -51,8 +51,7 @@ class Test extends AbstractModelsCommand
         ]);
 
         $this->assertSame(0, $tester->getStatusCode());
-        $this->assertEmpty($tester->getDisplay());
-
+        $this->assertStringContainsString('Written new phpDocBlock to', $tester->getDisplay());
         $this->assertMatchesPhpSnapshot($actualContent);
     }
 }