Fixed test break using file transport.

tests/unit/VendorTestCase.php

@@ -27,4 +27,4 @@ class VendorTestCase extends TestCase
 			throw new NotSupportedException("Vendor autoload file '{$vendorAutoload}' is missing.");
 		}
 	}
-}
+}

tests/unit/extensions/swiftmailer/MessageTest.php

@@ -56,7 +56,9 @@ class MessageTest extends VendorTestCase
 	 */
 	protected function createTestEmailComponent()
 	{
-		$component = new Mailer();
+		$component = new Mailer([
+			'useFileTransport' => true,
+		]);
 		return $component;
 	}