[Microbatch] Update tests to use microbatch behavior flag

[QMalcolm]

In #1334 we implemented the microbatch strategy. As part of that we setup the related tests to an enviornment variable for access the microbatch strategy, as that is what core had implemented. However, dbt-core is moving to using a project flag / behavior flag for this functionality instead of an environment variable (dbt-labs/dbt-core#10799). We need to update our tests to reflect this. The behavior flag is being added via dbt-adapters (dbt-labs/dbt-adapters#323).