Add tests for keywords

[khieta]

Describe the improvement you'd like to request

Add tests for the use of keywords in Cedar and JSON formats to make sure that 1) the results are what we expect and 2) the parser error messages make sense.

We should test:

• [x] Cedar policy format
• [ ] JSON policy format
• [ ] Cedar schema format
• [ ] JSON schema format
• [ ] JSON entity/context format

Describe alternatives you've considered

No response

Additional context

No response

Is this something that you'd be interested in working on?

• [ ] 👋 I may be able to implement this internal improvement
• [ ] ⚠️ This feature might incur a breaking change

[khieta]

Cedar policy tests added in #1223. Tests include the keywords "true", "false", "if", "then", "else", "in", "like", "has", "is", "__cedar", "principal", "action", "resource", "context", "permit", "forbid", "when", "unless".

[john-h-kastner-aws]

ref #407 for "parser error messages make sense."