Add option to run pytest unit tests from dbt-build

[andrewmogan]

At the last software coordination meeting (Dec. 2024), it was requested that we run pytest unit tests as part of the nightly workflows. I imagine this would work very similarly to dbt-build --unittest in terms of finding and collecting tests, but since --unittest specifically looks for C++ tests, I think this will need to be a separate option, like dbt-build --pytest.