tests: verify that the book URL redirects work

Now that `external/book/` has been re-generated, we can safely verify in
the CI builds that these redirects keep working.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>

tests/git-scm.spec.js

@@ -199,7 +199,11 @@ test('manual pages', async ({ page }) => {
 })
 
 test('book', async ({ page }) => {
+  await page.goto(`${url}book/`)
+  await expect(page).toHaveURL(`${url}book/en/v2`)
+
   await page.goto(`${url}book`)
+  await expect(page).toHaveURL(`${url}book/en/v2`)
 
   // Navigate to the first section
   await page.getByRole('link', { name: 'Getting Started' }).click()