TestClass and TestClassInstance own the management of their contained CaseExecution[] arrays, exposing them as IReadOnlyList<CaseExecution>.
Simplify TestClassInstance by letting it own the execution of the convention's case behavior against any given case.
Revert "Simplify TestClassInstance by letting it own the execution of the convention's case behavior against any given case."

This reverts commit d019a42148fa6ebd01c38e1f05c1fe310385d8d5.
Improve consistency between TestClass and TestClassInstance by giving both of them a reference to the Convention they are running under.