test(c3): use an Astro page to test preview

[ascorbic]

Updates the c3 Astro worker test to use an Astro page rather than an endpoint. This allows it to catch errors in rendering such as #10855

• Tests
  • Tests included/updated
  • Automated tests not possible - manual testing has been completed as follows:
  • Additional testing not necessary because: just tests
• Public documentation
  • Cloudflare docs PR(s):
  • Documentation not necessary because: just tests

A picture of a cute animal (not mandatory, but encouraged)

[changeset-bot]

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Latest commit: 96cbf22

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[vicb]

Thanks!