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.