Created a Mocks directory to seperate mocks from classes.
I moved Mailgun\Tests\MailGunTest to Mailgun\Test\Mocks\Mailgun. As it's not a
test it's a mock. So shouldn't be suffixed with Test.
I mocked Mailgun\Tests\Connection\TestBroker to Mailgun\Tests\Mocks\Connection\TestBroker as it's a mock and should be with other mocks.