agent: Return problems for CockpitTextStream open failures

For the cases where the socket file is not found, return "not-found"
and when permission denied, return "not-authorized".

Fixes #266
Closes #270
agent: Skip 'not-authorized' check when running as root

Since root bypasses unix priveleges all together.
tap-gtester: If test says it's skipped ignore results

This is a change from the gnome-keyring project.