Tweak mail extender (tests)

- Use private over protected
- Use "public" API for building requests in tests
- Add more assertions
- Formatting
- Use correct parameter order for assertions

Refs #2012.

src/Extend/Mail.php

@@ -14,7 +14,7 @@ use Illuminate\Contracts\Container\Container;
 
 class Mail implements ExtenderInterface
 {
-    protected $drivers = [];
+    private $drivers = [];
 
     /**
      * Add a mail driver.