servertest: fail step checks that never hold

A match on the last sample before the deadline no longer passes.

hscontrol/servertest/lifecycle_compat_test.go

@@ -349,6 +349,11 @@ func holdChecks(
 		}
 	}
 
+	// A last-sample match is not a settled one.
+	if held < holdTicks {
+		t.Errorf("%s: checks still changing at the deadline", strings.TrimSuffix(prefix, "/"))
+	}
+
 	for i, c := range checks {
 		key := prefix + c.name
 		evaluated[key] = true

hscontrol/servertest/via_ha_compat_test.go

@@ -871,7 +871,9 @@ func (r *stepReplay) apply(t *testing.T, step testcapture.Step) map[string]error
 			}
 
 		case "wait":
-			// SaaS settle time.
+			// SaaS settle time. The test server runs no scheduled tasks
+			// (expiry, HA probes), so passing time alone changes nothing
+			// but ephemeral collection, which its consumers shorten.
 
 		default:
 			t.Fatalf("%s: no replay for action %q", step.Name, a.Kind)