groundwork for typecheck test utils to accept Cedar schema syntax

[cdisselkoen]

Description of changes

Adjusts the utilities in typecheck/test to, in the future, accept Cedar schema syntax in addition to various other structures that were already accepted.

Issue #, if available

Checklist for requesting a review

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

• [x] 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):

• [x] 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):

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