[6.x] Support Partial Mocks on Facades

[hettiger]

Suppose we have a custom Facade called SomeService and want to mock just a single method.
This situation is usually leading to an error as follows:

SomeService::shouldReceive('foo')->once();
SomeService::foo();
SomeService::bar(); // Mockery\Exception\BadMethodCallException : Received Mockery_2_App_SomeService::bar(), but no expectations were specified 

My patch adds support for partial mocks in two ways:

SomeService::shouldReceive('foo')->once();
SomeService::partialMock();
SomeService::foo(); // this call is mocked
SomeService::bar(); // this call is deferred to it's parent

SomeService::partialMock()->shouldReceive('foo')->once();
SomeService::foo(); // this call is mocked
SomeService::bar(); // this call is deferred to it's parent

It is already possible to do something very similar relying on undocumented behavior:

SomeService::shouldReceive('foo')->once();
SomeService::makePartial();
SomeService::foo(); // this call is mocked
SomeService::bar(); // this call is deferred to it's parent

However my IDE is yelling at me and I strongly prefer a supported option if possible.
Also the above workaround is not as flexible:

SomeService::makePartial()->shouldReceive('foo')->once(); // Error : Call to undefined method SomeService::makePartial()
SomeService::foo();
SomeService::bar();