Add acceptance tests for how provider handles request_reason argument

[modular-magician]

This PR:

• test how the provider behaves when provider configuration arguments come from different sources (config vs ENVs)
• schema-level validation that's in place, e.g. handling of empty strings
  • See note below
• use cases: how does this argument impact the providers behaviour in plan/apply
  • This cannot be tested as the X-Goog-Request-Reason header is for auditing purposes in GCP

Writing these tests made me realise that empty string validation doesn't cover request_reason, so I opened: hashicorp/terraform-provider-google#19643 . The acceptance tests are written to define the current behaviour and my expectation is that the PF schema will be update in future to add validation, make those tests fail, and the tests will be updated.

Release Note Template for Downstream PRs (will be copied)

Derived from GoogleCloudPlatform/magic-modules#11835