Fixes coder/internal#272

• Increases healthcheck timeout in tests. This seems to be the most usual cause of test failures.
• Adds a non-nilness check before caching a healthcheck report.
• Modifies the HTTP response code to 503 (was 404) when no healthcheck report is available. 503 seems to be a better indicator of the server state in this case, whereas 404 could be misinterpreted as a typo in the healthcheck URL.