Use defer_relation for unit test type detection if available

[jtcohen6]

needed for dbt-labs/dbt-core#9199

Problem

dbt-labs/dbt-core#9199
moves deferral logic to RefResolver. But it won't apply for unit test fixtures because they are materialized ephemeral.

Solution

Within get_fixture_sql itself, use defer_relation if available, then fall back to this.

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