Support Dafny tests for benerated test models

[robin-aws]

We've been manually editing Rust source tests, which is also valuable testing, but it's also very easy to test for the wrong symbols.

Primary blocker is figuring out the target for the Dafny tests - they are translated into a single main method instead of individual [#test]s. What's the most idiomatic way to provide a main method that should only be used for testing a library and not called by consumers?