[5.6] Return instance of spy when swapping facade for a Mockery spy

[dallincoons]

One of the main benefits of spies is that you can move the assertion to the end of the test method. For example, instead of

$mock->shouldReceive('foo')->once()
//act on something

after the 'act' step in your test, you can do

// act on something
$spy->shouldHaveRecieved('foo')->once()

Right now, if I want to get an instance of the spy I have to do
$spy = Facade::getFacadeRoot()
so that I can use the spy in my assertions at the end of the test, which isn't as nice as it could be. This PR simply returns the spy after creating it.

[sisve]

I see you're changing an existing release (5.6). Could these changes cause existing code or packages to fail?

[dallincoons]

It behaves the same but just has a return value now.

[taylorotwell]

Why would you need an instance of the Spy? Can't you just do WhateverFacade::shouldHaveReceived?