test: the cookie signer and test modules read the never-back clock too

The docker VM's clock steps back: itsdangerous refused a cookie signed in the next second (a
random 401 right after a sign-in), and a test compared an app stamp with the raw wall clock.

backend/tests/test_test_clock.py

@@ -0,0 +1,51 @@
+"""The suite's clock (conftest `_clock_never_goes_back`), DB-lane.
+
+The wall clock of the docker VM the suite runs in steps BACK now and then (measured: up to 0.5 s).
+These hold the two places that read it besides the app: the session cookie's signature, and a test
+comparing a stamp the app wrote with "now".
+"""
+
+import time
+from datetime import datetime, timezone
+from types import SimpleNamespace
+
+import itsdangerous.timed
+
+_PW = "correct horse battery"
+
+
+def test_a_cookie_signed_just_before_the_clock_steps_back_still_signs_in(
+    client, make_user, monkeypatch
+):
+    # The signature carries whole seconds and itsdangerous refuses one from the future: a step back
+    # across a second boundary refused the cookie a sign-in had just set (a 401 at random).
+    clock = [int(time.time()) + 0.2]
+    monkeypatch.setattr(itsdangerous.timed, "time", SimpleNamespace(time=lambda: clock[0]))
+    make_user(email="clock@example.com", password=_PW)
+    r = client.post("/auth/password-login", json={"email": "clock@example.com", "password": _PW})
+    assert r.status_code == 200
+    clock[0] -= 0.5
+    assert client.get("/api/me/notifications").status_code == 200
+
+
+def test_a_test_reads_the_clock_the_app_stamps_with(monkeypatch):
+    # A test asserting "the app's stamp is not in the future" compared it with the wall clock, which
+    # may have stepped back since the app read its own.
+    import conftest
+    from app import diagnostics
+
+    # The app reads 12:00:01, then the wall clock steps back half a second (a time past any real one,
+    # so a test reading the wall clock instead is behind the stamp).
+    later = datetime(2099, 1, 1, 12, 0, 1, tzinfo=timezone.utc)
+    stepped_back = datetime(2099, 1, 1, 12, 0, 0, 500000, tzinfo=timezone.utc)
+    readings = iter([later])
+
+    class Stepping(datetime):
+        @classmethod
+        def now(cls, tz=None):
+            return next(readings, stepped_back)
+
+    monkeypatch.setattr(conftest._NeverBack, "clock", Stepping)
+    monkeypatch.setattr(conftest._NeverBack, "last", None)
+    stamped = diagnostics.datetime.now(timezone.utc)
+    assert datetime.now(timezone.utc) >= stamped