tailcfg, control/controlclient: add goroutine dump debug feature

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

control/controlclient/debug_test.go

@@ -0,0 +1,11 @@
+// Copyright (c) 2021 Tailscale Inc & AUTHORS All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package controlclient
+
+import "testing"
+
+func TestScrubbedGoroutineDump(t *testing.T) {
+	t.Logf("Got:\n%s\n", scrubbedGoroutineDump())
+}