net/interfaces: add IsExpensive and up state to State

Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

net/interfaces/interfaces_test.go

@@ -27,5 +27,23 @@ func TestIsTailscaleIP(t *testing.T) {
 			t.Errorf("F(%q) = %v; want %v", tt.ip, got, tt.want)
 		}
 	}
-
+}
+
+func TestGetState(t *testing.T) {
+	st, err := GetState()
+	if err != nil {
+		t.Fatal(err)
+	}
+	t.Logf("Got: %#v", st)
+
+	st2, err := GetState()
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	if !st.Equal(st2) {
+		// let's assume nobody was changing the system network interfaces between
+		// the two GetState calls.
+		t.Fatal("two States back-to-back were not equal")
+	}
 }