tests/gem_flink: Check for MASTER before proceeding

gem_flink|gem_open are DRM_AUTH ioctl, and being lazy we do not
establish ourselves as authenticated before testing the ioctls. So
instead of aborting, skip the test unless we have root privileges (and
so DRM_MASTER and the DRM_AUTH restriction no longer applies).

A future test could assert that the flink fails without proper
authentication.

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=43924


Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>