Move tests out of api.rs

[andrewmwells-amazon]

Description of changes

Move tests out of api.rs. IMO this is cleaner than having almost 3k lines of tests in the public API.

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The change in this PR is (choose one, and delete the other options):

• A change "invisible" to users (e.g., documentation, changes to "internal" crates like cedar-policy-core, cedar-validator, etc.)

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• Does not update the CHANGELOG because my change does not significantly impact released code.

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• Does not require updates because my change does not impact the Cedar Dafny model or DRT infrastructure.

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[shaobo-he-aws]

LGTM. That being said, is there a convention where we want to put unit tests (e.g., a dedicated folder)?

[cdisselkoen]

stupid question, did you do the simple check that the tests are still running? (like, just break one and check.) I'm a little surprised we didn't need mod tests; anywhere

[andrewmwells-amazon]

LGTM. That being said, is there a convention where we want to put unit tests (e.g., a dedicated folder)?

I just followed e.g., https://github.com/rust-lang/rust/blob/master/compiler/rustc_graphviz/src/tests.rs here. I don't know if a one-size-fits-all strategy makes the most sense.

[andrewmwells-amazon]

stupid question, did you do the simple check that the tests are still running? (like, just break one and check.) I'm a little surprised we didn't need mod tests; anywhere

We did in fact need a mod tests;

[andrewmwells-amazon]

Fails correctly with broken unit test: https://github.com/cedar-policy/cedar/actions/runs/6897658266/job/18766238211?pr=446

[john-h-kastner-aws]

I like it.