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>

ipn/localapi/localapi.go

@@ -557,6 +557,13 @@ func (h *Handler) serveDebug(w http.ResponseWriter, r *http.Request) {
 		err = h.b.DebugBreakTCPConns()
 	case "break-derp-conns":
 		err = h.b.DebugBreakDERPConns()
+	case "control-knobs":
+		k := h.b.ControlKnobs()
+		w.Header().Set("Content-Type", "application/json")
+		err = json.NewEncoder(w).Encode(k.AsDebugJSON())
+		if err == nil {
+			return
+		}
 	case "":
 		err = fmt.Errorf("missing parameter 'action'")
 	default: