test: improve message in immutable cache test

Author: mrstork

tests/utils/playwright-helpers.ts

@@ -98,7 +98,7 @@ export const test = base.extend<
  if (response.url().includes('/_next/static/')) {
  expect(
  response.headers()['cache-control'],
- '_next/static assets should have immutable cache control',
+ `_next/static asset (${response.url()}) should have immutable cache control`,
  ).toContain('public,max-age=31536000,immutable')
  }
  })