review classifier: a test-only diff satisfies tests-with-behaviour

The rule asked for a test beside every behaviour change and had no
answer for a diff that is itself a test. On #5207, whose only file is the
module-eval fixture nix/module-eval/agent-queue-bao.nix, it answered
"no match" at certainty 78 and requested changes.

Name the test locations of this repo and make the question not
applicable when every changed file is one of them. A PR that changes
behaviour and tests together still matches, and a behaviour change with
no test still fails.

.hyperhive/review-classifier.yml

@@ -34,7 +34,11 @@ classifications:
   - name: tests-with-behaviour
     description: >-
       Every change to how the code behaves comes with a new or updated test
-      that exercises it.
+      that exercises it. A diff whose changed files are all test code
+      (a fixture under `nix/module-eval/`, `nix/checks.nix`, a crate's
+      `tests/` directory, or Rust inside `#[cfg(test)]` or a `#[test]`
+      function) changes no behaviour: the question is not applicable. A PR
+      that changes behaviour and tests together matches.
     polarity: positive
     threshold: 65
   - name: docs-updated