dev: introduced FuncTestFixtureFactory.
dev: implemented the Func property.
dev: added null guard clause to the constructor
dev: implemented the Create Method.
refactor: used FuncTestFixtureFactory in FirstClassTestAttribute.
docs: added release note for FuncTestFixtureFactory.
docs: removed unnecessary comment.
test: made ExperimentUnitTest don't use NewAppDomainFact.
test: made Experiment.XunitUnitTest don't use NewAppDomainFact.
test: removed the unused classes.