Introduce a gate/check GHA job

This adds a GHA job that reliably determines if all the required
dependencies have succeeded or not.

It also allows to reduce the list of required branch protection CI
statuses to just one — `check`. This reduces the maintenance burden
by a lot and have been battle-tested across a small bunch of projects
in its action form and in-house implementations of other people.

I was curious about the spread of use. And I've just discovered that
it is now in use in aiohttp (and other aio-libs projects), CherryPy,
some of the Ansible repositories, all of the jaraco's projects (like
`setuptools`, `importlib_metadata`), some PyCQA, PyCA and pytest
projects, a few AWS Labs projects. Admittedly, I maintain a few of
these but it seems to address some of the pain folks have:
https://github.com/jaraco/skeleton/pull/55#issuecomment-1106638475.

The story behind this is explained in more detail at
https://github.com/marketplace/actions/alls-green#why.

.github/workflows/test.yml

@@ -211,3 +211,17 @@ jobs:
         fail_ci_if_error: true
         files: ./coverage.xml
         verbose: true
+
+  check:  # This job does nothing and is only used for the branch protection
+    if: always()
+
+    needs:
+    - build
+
+    runs-on: ubuntu-latest
+
+    steps:
+    - name: Decide whether the needed jobs succeeded or failed
+      uses: re-actors/alls-green@release/v1
+      with:
+        jobs: ${{ toJSON(needs) }}