igt_command_line.sh: Abort if run as root

`ninja test` invokes igt_command_line.sh for each test binary to check
the behavior of some of the switches. One of the verified things is that
the test exists with non-zero status when requesting invalid subtest.

`--run-subtest` results in igt_fixtures being executed - this fiddles
with the device and sysfs knobs.

Let's exit early, if we are root, to save people form unintentional
side-effects and strange failures.

Cc: Petri Latvala <petri.latvala@intel.com>
Cc: Andi Shyti <andi.shyti@intel.com>
Signed-off-by: Arkadiusz Hiler <arkadiusz.hiler@intel.com>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>