test: render helm templates before filtering manifests for debugging

Summary:
There are a class of Helm errors that are obscured with cryptic
error messages if --show-only is enabled.

To get around this, render all the templates. If that command succeeds,
then add --show-only.

Test Plan: N/A

Reviewers: johann, jordansanders, sidkmenon

Reviewed By: jordansanders

Differential Revision: https://dagster.phacility.com/D8327