tests/netgraph: mark all tests as required_user="root"

tests/netgraph: mark all tests as required_user="root"

Any netgraph operation requires root priveleges.  Some tests in the
directory already mark themselves with 'atf_tc_set_md_var(conf,
"require.user", "root");' which creates a lot of pasted code.  Some tests
don't mark self.  For this particular directory a blanket metadata setting
in the Makefile is acceptable, imho.