ipn: close logger at the end of TestLocalLogLines

If any goroutine continues to use the logger in TestLocalLogLines
after the test finishes, the test panics.

The culprit for this was wireguard-go; the previous commit fixed that.
This commit adds suspenders: When the test is done, make logging calls
into no-ops.

Signed-off-by: Josh Bleecher Snyder <josh@tailscale.com>

ipn/loglines_test.go

@@ -26,6 +26,7 @@ func TestLocalLogLines(t *testing.T) {
 		"[v1] peer keys: %s",
 		"[v1] v%v peers: %v",
 	})
+	defer logListen.Close()
 
 	logid := func(hex byte) logtail.PublicID {
 		var ret logtail.PublicID