refactor: use golden files to store test data for integraiton tests

nit: would it help if we have some _YAML files with test input data?_ I mean, less Go code...

_Originally posted by @mtojek in https://github.com/coder/terraform-provider-envbuilder/pull/44#discussion_r1743231711_

Start by reviewing the integration tests and the discussion in pull request #44, since no specific files or test cases are named here. The intended result appears to be storing integration-test input data in YAML golden files instead of Go code; confirm the scope and expected format before changing anything.

Written by the indexing model from the issue text.