Verify Element X with an existing Element Classic account. (#5374)

* Read and import the secrets from ClassicAppAccounts.

* Record snapshots.

* Add some documentation, tidy up tests and fix the dismissal of the backup instructions.

* Workaround flakey tests (the fulfilments weren't always firing).

* Allow a custom Classic App deep link URL to be configured.

ElementX/Sources/Mocks/AuthenticationClientBuilderFactoryMock.swift

@@ -14,6 +14,7 @@ extension AuthenticationClientFactoryMock {
     struct Configuration {
         var homeserverClients = [
             "matrix.org": ClientSDKMock(configuration: .init()),
+            "https://matrix-client.matrix.org": ClientSDKMock(configuration: .init()),
             "example.com": ClientSDKMock(configuration: .init(serverAddress: "example.com",
                                                               homeserverURL: "https://matrix.example.com",
                                                               slidingSyncVersion: .native,
@@ -51,5 +52,12 @@ extension AuthenticationClientFactoryMock {
             }
             return client
         }
+        
+        makeInMemoryClientHomeserverAddressClientSessionDelegateAppSettingsAppHooksClosure = { address, _, _, _ in
+            guard let client = configuration.homeserverClients[address] else {
+                throw ClientBuildError.ServerUnreachable(message: "Not a known homeserver.")
+            }
+            return client
+        }
     }
 }