cmd/tailscaled,net/tstun: fix data race on start-up in TUN mode

Fixes #7894

Change-Id: Ice3f8019405714dd69d02bc07694f3872bb598b8

Co-authored-by: Brad Fitzpatrick <bradfitz@tailscale.com>
Signed-off-by: Maisem Ali <maisem@tailscale.com>

net/tstun/wrap_test.go

@@ -178,6 +178,7 @@ func newChannelTUN(logf logger.Logf, secure bool) (*tuntest.ChannelTUN, *Wrapper
 	} else {
 		tun.disableFilter = true
 	}
+	tun.Start()
 	return chtun, tun
 }