ipn/ipnlocal: remove NewLocalBackendWithClientGen

This removes the NewLocalBackendWithClientGen constructor added in
b4d04a065f and instead adds
LocalBackend.SetControlClientGetterForTesting, mirroring
LocalBackend.SetHTTPTestClient. NewLocalBackendWithClientGen was
weird in being exported but taking an unexported type. This was noted
during code review:

https://github.com/tailscale/tailscale/pull/1818#discussion_r623155669

which ended in:

"I'll leave it for y'all to clean up if you find some way to do it elegantly."

This is more idiomatic.

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

ipn/ipnlocal/state_test.go

@@ -278,7 +278,11 @@ func TestStateMachine(t *testing.T) {
 	}
 
 	cc := newMockControl()
-	ccGen := func(opts controlclient.Options) (controlclient.Client, error) {
+	b, err := NewLocalBackend(logf, "logid", store, e)
+	if err != nil {
+		t.Fatalf("NewLocalBackend: %v", err)
+	}
+	b.SetControlClientGetterForTesting(func(opts controlclient.Options) (controlclient.Client, error) {
 		cc.mu.Lock()
 		cc.opts = opts
 		cc.logf = opts.Logf
@@ -289,11 +293,7 @@ func TestStateMachine(t *testing.T) {
 		cc.logf("ccGen: new mockControl.")
 		cc.called("New")
 		return cc, nil
-	}
-	b, err := NewLocalBackendWithClientGen(logf, "logid", store, e, ccGen)
-	if err != nil {
-		t.Fatalf("NewLocalBackend: %v", err)
-	}
+	})
 
 	notifies := &notifyThrottler{t: t}
 	notifies.expect(0)