add fakeit mocking header library for unit tests

[scravy]

Extracted from #137

This includes the header-only fakeit library into the unit test target (so no dependencies added to united etc.). fakeit can mock interfaces (anything which std::is_polymorphic yields true for ;-)).

Here are usage examples extracted from their README:

class SomeInterface {
	virtual int foo(int) = 0;
	virtual int bar(string) = 0;
};

// Instantiate a mock object.
Mock<SomeInterface> mock;

// Setup mock behavior.
When(Method(mock,foo)).Return(1); // Method mock.foo will return 1 once.

// Fetch the mock instance.
SomeInterface &i = mock.get();

// Will print "1". 
cout << i.foo(0);

Verify method invocation

Mock<SomeInterface> mock;
		
When(Method(mock,foo)).Return(0);

SomeInterface &i = mock.get();

// Production code
i.foo(1);

// Verify method mock.foo was invoked.
Verify(Method(mock,foo));

// Verify method mock.foo was invoked with specific arguments.
Verify(Method(mock,foo).Using(1));

[AM5800]

I think it is cool to have a mocking library.
utACK

[kostyantyn]

ConceptACK

[Gnappuraz]

❤️ utACK bfe6917

[Ruteri]

utACK, very cool.
The linter failed and we probably want to exclude the file.

[scravy]

@Ruteri The linter failed on whitespace in the README.