add tests for reserved keywords in policies

[khieta]

Description of changes

Adds some tests to check when keywords are allowed/not allowed in policies. I was using these to check whether our docs were up-to-date, but figured they might be useful to add to the repo so we can track changes. I found this a useful exercise, which led me to open #1221, and I think we should do this for more formats (#1222).

Issue #, if available

Checklist for requesting a review

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.)

I confirm that this PR (choose one, and delete the other options):

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

I confirm that cedar-spec (choose one, and delete the other options):

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

I confirm that docs.cedarpolicy.com (choose one, and delete the other options):

• Does not require updates because my change does not impact the Cedar language specification.

[john-h-kastner-aws]

tests <3