Check that all omissions from tests-pass are intentional

This intentionally fails right now, by omitting `tests-pass` itself
as a job that `tests-pass` should not depend on, in order to ensure
that it is able to fail. Once this is observed (and any other bugs
fixed), this omission should be corrected, and then it should pass.

.github/workflows/ci.yml

@@ -289,6 +289,42 @@ jobs:
           git status
           git diff --exit-code
 
+  # Check that only jobs intended not to block PR auto-merge are omitted as
+  # dependencies of the `tests-pass` job below, so that whenever a job is
+  # added, a decision is made about whether it must pass for PRs to merge.
+  check-blocking:
+    runs-on: ubuntu-latest
+
+    env:
+      # List all jobs that are intended NOT to block PR auto-merge here.
+      # FIXME: After ensuring this can fail, add `test-pass` below to fix it.
+      EXPECTED_NONBLOCKING_JOBS: |-
+        test-fixtures-windows
+      #  tests-pass
+
+    defaults:
+      run:
+        shell: bash  # Without specifying this, we don't get `-o pipefail`.
+
+    steps:
+      - name: Find this workflow
+        run: |
+          relative_workflow_with_ref="${GITHUB_WORKFLOW_REF#"$GITHUB_REPOSITORY/"}"
+          echo "WORKFLOW_PATH=${relative_workflow_with_ref%@*}" >> "$GITHUB_ENV"
+      - uses: actions/checkout@v4
+        with:
+          sparse-checkout: ${{ env.WORKFLOW_PATH }}
+      - name: Get all jobs
+        run: yq '.jobs | keys.[]' -- "$WORKFLOW_PATH" | sort > all-jobs.txt
+      - name: Get blocking jobs
+        run: yq '.jobs.tests-pass.needs.[]' -- "$WORKFLOW_PATH" | sort > blocking-jobs.txt
+      - name: Get jobs we intend do not block
+        run: sort <<<"$EXPECTED_NONBLOCKING_JOBS" > expected-nonblocking-jobs.txt
+      - name: Check that each job blocks PRs xor intentionally doesn't
+        run: |
+          sort -m blocking-jobs.txt expected-nonblocking-jobs.txt |
+            diff --color=always -U1000 -- - all-jobs.txt
+
   # Dummy job to have a stable name for the "all tests pass" requirement
   tests-pass:
     name: Tests pass
@@ -302,6 +338,7 @@ jobs:
       - cargo-deny
       - wasm
       - check-packetline
+      - check-blocking
 
     if: always() # always run even if dependencies fail