add test case when raise ServiceUnavailable in is_retryable

[jx2lee]

(want to) resolves #682
docs dbt-labs/docs.getdbt.com/#

Problem

some users claims not to retry on BQ 503 errors.

Solution

Add case to the test_is_retryable raises a ServiceUnavailable error.

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