Easier payment testing (#2672)

* Easier payment testing

* WIP, more TODOs and some cleanup. Help is appreciated.

* Added dummy button to expire monitoring (doesn't work yet)

* Added TODO

* Make fake tab default if present

* Split controller and change wording from fake to testing

* Extract and simplify checkout testing UI

* Restrict testing access to regtest

Co-authored-by: Dennis Reimann <mail@dennisreimann.de>

BTCPayServer.Client/Models/FakePaymentRequest.cs

@@ -0,0 +1,9 @@
+using System;
+
+namespace BTCPayServer.Client.Models
+{
+    public class FakePaymentRequest
+    {
+        public Decimal Amount { get; set; }
+    }
+}