dev: implements TestFixtureFactoryAttribute.
dev: implements TestAttribute.
dev: implements FirstClassTestAttribute.
dev: implements AutoFixtureFactory.
refactor: renames TestFixtureFactoryAttribute to

TestFixtureDeclarationAttribute.
dev: deletes FirstClassTheoremAttribute.
dev: deletes TheoremAttribute.
dev: deletes FixtureFactory.
dev: deletes TheoremBaseAttribute.
dev: deletes FirstClassTheoremBaseAttribute.
refactor: renames test-attributes in scenario tests to clarify.
refactor: slightly reformats.
build: addresses code-analysis warnings.
docs: adds release note for the model of test fixture declaration.
refactor: renames a private method to clarify.
dev: makes creating singleton of TestFixtureFactory thread-safe.
refactor: improves running tests with shared(static) fixture.
build: creates the solution file for All projects and items.
build: renames the solution file 'SolutionItems' to 'Solution Items'.
build: removes the ExperimentUnitTest project from specific solution

files.