[Snowflake] test types for unit testing

[MichelleArk]

resolves #898
resolves #891
docs dbt-labs/docs.getdbt.com/#

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Solution

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• 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, or tests are not required/relevant for this PR
• This PR has no interface changes (e.g. macros, cli, logs, json artifacts, config files, adapter interface, etc) or this PR has already received feedback and approval from Product or DX

[github-actions]

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[mikealfare]

The two unrelated, failed test cases on the dbt version was an issue in dbt-tests-adapter which has since been resolved. However it looks like a relevant test case is failing as well.

[jtcohen6]

Based on some quick local testing 🎩 with this branch, I think we need to incorporate some of the logic from #839 for the unstructed (variant/array/object) types — but I could be mistaken! (especially since I'm having trouble running the adapter functional tests locally)

That PR also included an update that mapped FIXED → NUMERIC types. I don't remember exactly why it was needed for unit tests, but it did have the nice side-effect of a clearer error message for model contract type mismatches.