Add tests for cross-database cast macro

[dbeatty10]

resolves #84
docs: dbt-labs/docs.getdbt.com#5278

Problem

Having a cross-database cast macro has been suggested as a way to address a variety of things including, but not limited to:

• case-insensitive comparisons in unit testing, base unit testing test #55
• [fix] use cast macro in default__get_empty_schema_sql #165
• test constraints for geometry and geography types dbt-snowflake#969
• [CT-816] [Bug] Incremental materialization - cast column when changing column type dbt-core#5445 (comment)

Solution

We actually already added it in 5a50be7 within #55 !

But this PR adds some tests (just duplicated from the ones for safe_cast) and a changelog entry.

Checklist

• I have read the contributing guide and understand what's expected of me
• I have run this code in development, and it appears to resolve the stated issue
• This PR includes tests
• This PR has already received feedback and approval from Product or DX

[mikealfare]

Your choice as to removing the test implementation or keeping it.