Standard libraries should enforce a minimum testing coverage threshold

[robin-aws]

When we developed the libraries, we had a target of at least 80% runtime testing coverage. This is important in order to encourage actually compiling and running the code across the different backends to find issues, and indirectly encourage enough examples to expose issues such as unsatisfiable preconditions. We did not set up any mechanism to enforce this, however, partially because dafny test does not include any such feature, and it's difficult to enforce after the fact on the report.