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>

control/controlknobs/controlknobs.go

@@ -39,3 +39,19 @@ type Knobs struct {
 	// running in magicsock, even when idle.
 	ForceBackgroundSTUN atomic.Bool
 }
+
+// AsDebugJSON returns k as something that can be marshalled with json.Marshal
+// for debug.
+func (k *Knobs) AsDebugJSON() map[string]any {
+	if k == nil {
+		return nil
+	}
+	return map[string]any{
+		"DisableUPnP":         k.DisableUPnP.Load(),
+		"DisableDRPO":         k.DisableDRPO.Load(),
+		"KeepFullWGConfig":    k.KeepFullWGConfig.Load(),
+		"RandomizeClientPort": k.RandomizeClientPort.Load(),
+		"OneCGNAT":            k.OneCGNAT.Load(),
+		"ForceBackgroundSTUN": k.ForceBackgroundSTUN.Load(),
+	}
+}