build: install xunit.extensions to Experiment.IdiomsUnitTest.
dev: add assembly level tests.
dev: introduce CompositeTestCaseCollection.
dev: add guard clause to CompositeTestCaseCollection.
dev: implement CompositeTestCaseCollection.
dev: add guard clauses to ReflectionElementCollection.
dev: implement some properties of ReflectionElementCollection.
dev: implement ReflectionElementCollection.GetEnumerator.
dev: implement the TestCaseSet property.
refactor: rename IdiomaticTestCaseCollection -> IdiomaticTestCases.
refactor: rename ReflectionElementCollection to ReflectionElements.
refactor: rename CompositeTestCaseCollection to CompositeTestCases.
build: address some code-analysis warnings.