ipn/ipnlocal: fix missing controlknobs.Knobs plumbing

I missed connecting some controlknobs.Knobs pieces in 4e91cf20a8
resulting in that breaking control knobs entirely.

Whoops.

The fix in ipn/ipnlocal (where it makes a new controlclient) but to
atone, I also added integration tests. Those integration tests use
a new "tailscale debug control-knobs" which by itself might be useful
for future debugging.

Updates #9351

Change-Id: Id9c89c8637746d879d5da67b9ac4e0d2367a3f0d
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

cmd/tailscale/cli/debug.go

@@ -138,6 +138,11 @@ var debugCmd = &ffcli.Command{
 			Exec:      localAPIAction("break-derp-conns"),
 			ShortHelp: "break any open DERP connections from the daemon",
 		},
+		{
+			Name:      "control-knobs",
+			Exec:      debugControlKnobs,
+			ShortHelp: "see current control knobs",
+		},
 		{
 			Name:      "prefs",
 			Exec:      runPrefs,
@@ -915,3 +920,17 @@ func runPeerEndpointChanges(ctx context.Context, args []string) error {
 	fmt.Printf("%s", dst.String())
 	return nil
 }
+
+func debugControlKnobs(ctx context.Context, args []string) error {
+	if len(args) > 0 {
+		return errors.New("unexpected arguments")
+	}
+	v, err := localClient.DebugResultJSON(ctx, "control-knobs")
+	if err != nil {
+		return err
+	}
+	e := json.NewEncoder(os.Stdout)
+	e.SetIndent("", "  ")
+	e.Encode(v)
+	return nil
+}