Introduce Behaviors and Conventions namespaces for organization.  Rename ClassFixture to CreateInstancePerCase.
Conventions use CreateInstancePerCase as their default TypeBehavior, but this behavior can be replaced or augmented with any TypeBehavior.
The default convention includes fixture classes with no default constructor, because omitting them would cause them to be silently skipped while including them allows them to be discovered and then to fail with a clear explanation.
Convention is no longer abstract.  Test coverage for the behavior of a newly constructed 'empty' Convention.