Timeline: repair oldest/newest metrics if we fetch nothing

ts/test-node/helpers.ts

@@ -0,0 +1,17 @@
+// Copyright 2020 Signal Messenger, LLC
+// SPDX-License-Identifier: AGPL-3.0-only
+
+import { assert } from 'chai';
+
+export async function assertRejects(fn: () => Promise<unknown>): Promise<void> {
+  let err: unknown;
+  try {
+    await fn();
+  } catch (e) {
+    err = e;
+  }
+  assert(
+    err instanceof Error,
+    'Expected promise to reject with an Error, but it resolved'
+  );
+}