tests: Ensure the S3 signed URLs used for Groups access are temporall…

…y stable

This is a carefully arranged property of them which I'd manually
validated when initially implementing.  As I have to change the
implementation to upgrade from the v2 to v3 AWS SDK, I want to
revalidate this property remains intact.

test/s3.test.js

@@ -0,0 +1,32 @@
+import { jest } from "@jest/globals";
+import { Group } from "../src/groups.js";
+
+
+afterEach(() => {
+  jest.restoreAllMocks();
+});
+
+
+test("signed URLs are temporally stable and thus cacheable", async () => {
+  const blab = new Group("blab");
+
+  const narrativeMarkdown =
+    blab.source
+      .narrative("test/fixture".split("/"))
+      .subresource("md");
+
+  const now = jest.spyOn(Date, "now");
+  const time = 1697737617069; // Thu Oct 19 10:46:57.069 2023 America/Los_Angeles
+
+  now.mockReturnValue(time);
+  const url1 = await narrativeMarkdown.url();
+
+  now.mockReturnValue(time + 10 * 60 * 1000); // 10 minutes later (in ms)
+  const url2 = await narrativeMarkdown.url();
+
+  now.mockReturnValue(time + 120 * 60 * 1000); // 120 minutes (2 hours) later (in ms)
+  const url3 = await narrativeMarkdown.url();
+
+  expect(url1).toStrictEqual(url2);
+  expect(url1).not.toStrictEqual(url3);
+});