Add test coverage for the userinfo profile-fetch fallback #620

The userinfo request in GIDSignIn.m does not have any test coverage (this is the fallback that fetches basic profile data if the ID token doesn't carry it).

Suggested changes:

• Set up a sign in whose ID token lacks profile claims, so the profile-fetch fallback runs
• Assert the userinfo request URL: scheme, host, path /oauth2/v3/userinfo, and the access_token query parameter
• Include a token containing URL-reserved characters (&, =, #). (testDisconnectNoCallback_tokenWithReservedCharacters does this to test recent changes that ensure those characters are encoded correctly)
• Assert the returned data maps correctly onto GIDProfileData