More integration test refactoring

[khieta]

Description of changes

• While working on Move ImpossiblePolicy from error to warning #716 I noticed that the cedar-policy integration tests weren't actually running after Refactoring in integration testing code #707 (oops). Added a main file to fix this.
• Added a ValidationComparisonMode to CedarTestImplementation that controls how validation results are used during testing. This should allow me to easily fix the CI failure in Move ImpossiblePolicy from error to warning #716, which is caused by validation differences in the corpus tests. I can also reuse this to clean up the implementation of cedar-spec#210, which updated the property tested in validation DRT.

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• A change (breaking or otherwise) that only impacts unreleased or experimental code.

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[aaronjeline]

tiny nits

[cdisselkoen]

any reason to have main.rs instead of lib.rs? this is a library crate, right?

[khieta]

No special reason, I was just copying the structure of the CLI tests. The files just contain #[test]s. Is lib or main more appropriate? Or something else?

[cdisselkoen]

lib.rs is used for crates that provide libraries, main.rs is used for crates compiling an executable. (main.rs usually contains a main() -- in fact, I'm surprised Cargo isn't complaining that this one doesn't.) See https://doc.rust-lang.org/cargo/guide/project-layout.html

[khieta]

Based on that link, I think that using a main.rs file in the tests subfolder is the correct structure.

If a binary, example, bench, or integration test consists of multiple source files, place a main.rs file along with the extra modules within a subdirectory of the src/bin, examples, benches, or tests directory. The name of the executable will be the directory name.

Also learned: the reason that the cedar-policy integration tests weren't running after #707 was the nested directory structure I introduced under tests/.

[khieta]

Fix for the CedarDRT build failure in cedar-spec#253.

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