Update test_incremental_microbatch to use behavior flag instead of environment variable

[QMalcolm]

resolves #1359

Problem

dbt-adapters + dbt-core are moving to a behavior flag for gating microbatch functionality. We need to update the tests in dbt-bigquery to reflet this.

Solution

Updated the effected tests to use the new behavior flag defined by dbt-adapters in dbt-labs/dbt-adapters#323

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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