Implement Matrix testing in CI

[jharley]

We currently run all integration tests with Terraform 1.0.11 (configured here) and haven't yet completed the work to enable parallel runs of the integration test suite against the same Honeycomb Team/Environment.

Ahead of cutting 1.0, testing with multiple X.Y.* versions of Terraform (and possibly OpenTofu) will allow us to ensure we're building features that work for the commonly used versions of Terraform (e.g. 1.[0-9].*).

This work is likely done in a few steps:

1. remove the need for the 'setup helper' bash script so that both the client and the provider integration tests pass
2. refactor the 'shape' of the CI work flow to look something like this:
   • lint+build
   • fanout to parallel US and EU tracks
     • run client tests for both regions in parallel
     • run integration tests for both regions in parallel
3. add a test matrix to the integration tests

HashiCorp has provided an example of a matrix pipeline for acceptance tests here.