Run the gitlab-tofu-ctl Go tests in CI

What

Adds a go:test:gitlab-tofu-ctl job that runs go test for src/gitlab-tofu-ctl.

Why

There was no go test job at all, so src/gitlab-tofu-ctl/cmd/auto_define_backend_test.go has never been executed by a pipeline since it was added. The rules and image mirror the neighbouring go:build:gitlab-tofu-ctl job so the tests run on the same set of changes.

Found while working on an unrelated feature branch that adds more Go tests; split out here so it can land on its own.