instant(): fix cookie handling for fresh page loads (#90613)
Previously, instant() used page.evaluate() to set the cookie via
document.cookie, which required a page to already be loaded. This meant
the cookie wasn't present on the very first navigation.
Switch to Playwright's browser context cookie API so the cookie is set
before any page loads. If the page is already loaded, the base URL is
inferred automatically. Otherwise, a new baseURL option allows setting
the cookie before the first navigation.
