add freezegun to dbt-tests-adapter dependencies

[MichelleArk]

resolves #N/A
docs dbt-labs/docs.getdbt.com/#

Problem

Should have gone in as part of #300, as downstream adapters will need freezegun installed to run th microbatch tests.

example failure: dbt-labs/dbt-snowflake#1179

Solution

Add freezegun as dependency of dbt-test-adapters with permissive pinning (risk of breakage from a dependency here is very minimal)

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, 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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