Omnipay Guzzle PHPUnit set/get mock json httpResponse

I'm new to OmniPay, playing around with it and trying to make a simple custom gateway, and create a unit test with a mock json http response.

In the GatewayTest.php I set a mock http response:

public function testPurchaseSuccess()
{
    $this->setMockHttpResponse('TransactionSuccess.txt');

    $response = $this->gateway->purchase($this->options)->send();

    echo $response->isSuccessful();

    $this->assertEquals(true, $response->isSuccessful());
}

In the PurchaseRequest.php I'm trying to get it somehow:

public function sendData($data)
{
    $httpResponse = //how do I get the mock http response set before?

    return $this->response = new PurchaseResponse($this, $httpResponse->json());
}

So how do I get the mock http response in PurchaseRequest.php?