iotests: Skip read-only cases in 118 when run as root

iotests: Skip read-only cases in 118 when run as root

Some tests in 118 use chmod to remove write permissions from the file
and assume that the image can indeed not be opened read-write
afterwards. This doesn't work when the test is run as root, because root
can still open the file as writable even when the permission bit isn't
set.

Introduce a @skip_if_root decorator and use it in 118 to skip the tests
in question when the script is run as root.

Signed-off-by: Kevin Wolf <kwolf@redhat.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>