net/netmon: use Monitor's tsIfName if set by SetTailscaleInterfaceName

Currently nobody calls SetTailscaleInterfaceName yet, so this is a
no-op. I checked oss, android, and the macOS/iOS client. Nobody calls
this, or ever did.

But I want to in the future.

Updates #15408
Updates #9040

Change-Id: I05dfabe505174f9067b929e91c6e0d8bc42628d7
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

net/netmon/interfaces_test.go

@@ -13,7 +13,7 @@ import (
 )
 
 func TestGetState(t *testing.T) {
-	st, err := getState()
+	st, err := getState("")
 	if err != nil {
 		t.Fatal(err)
 	}