Run netkit test in separate network namespace

[ti-mo]

This blew up on my machine today:

--- FAIL: TestNetkitAnchor (0.00s)
    quicktest.go:12:
        error:
          got non-nil value
        got:
          e"netlink receive: file exists"
        stack:
          /home/timo/github/cilium/ebpf/link/netkit_test.go:95
            qt.Assert(tb, qt.IsNil(err))
          /home/timo/github/cilium/ebpf/link/netkit_test.go:32
            linkA, ifIndex := mustAttachNetkit(t, a, ebpf.AttachNetkitPrimary)

FAIL

This test should be running in a new netns so it doesn't interfere with the host's network stack.