Disable the Digest Authentication tests for iOS and tvOS.
Disable the Digest Authentication tests for iOS and tvOS.

They started failing a while ago, so get them out of the way of automated
tests.  There is an issue opened for follow up.

google/gtm-session-fetcher,thomasvl/gtm-session-fetcher,google/gtm-session-fetcher,thomasvl/gtm-session-fetcher,thomasvl/gtm-session-fetcher,google/gtm-session-fetcher