TeamCity: Add GHAs to run the TeamCity configuration's tests on PRs editing the TeamCity configuration

[SarahFrench]

Background

There are automated tests for the TeamCity configuration but they can only be run locally by developers. We should automate running these tests to prevent bugs entering the code.

The Azure provider has pre-existing GHAs for running TeamCity tests, so we can take a lot of inspiration from them:

• GHA workflow file: .github/workflows/teamcity-test.yaml
• Makefile target used to support that workflow: teamcity-test

We should make this GHA run on PRs where files in mmv1/third_party/terraform/.teamcity are edited.

What kind of contribution is this issue about?

• Contribution to the TeamCity config

Related PR(s), if any:

• Update GNUmakefile to include teamcity-test target #17223

Details

We don't need to generate the downstream provider to run these tests - we can run them directly in mmv1/third_party/terraform/.teamcity. This will continue to be true as long as those files aren't compiled (currently they're all copied unchanged into the TPG repo, and not TPGB).

[SarahFrench]

This issue is like a cousin of #17168, but neither blocks the other.

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