tests: Skip 'set-network-interface-address' test when running as root.

Reported by Eric Hanchrow <eric.hanchrow@gmail.com>
in <http://bugs.gnu.org/21354>.

* tests/syscalls.scm ("set-network-interface-address"): Skip when (getuid)
  returns zero.

tests/syscalls.scm

@@ -197,6 +197,7 @@
     (close-port sock)
     addr))
 
+(test-skip (if (zero? (getuid)) 1 0))
 (test-assert "set-network-interface-address"
   (let ((sock (socket AF_INET SOCK_STREAM 0)))
     (catch 'system-error