The type used for functions that operate on test fixtures. This is used for the fixture setup and teardown functions as well as for the testcases themselves.
@user_data is a pointer to the data that was given when registering the test case.
@fixture will be a pointer to the area of memory allocated by the test framework, of the size requested. If the requested size was zero then @fixture will be equal to @user_data.
Pointer(Void) -> Nil