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/SDK/ClientSDKMock.swift

@@ -46,11 +46,16 @@ extension ClientSDKMock {
         serverReturnValue = "https://\(configuration.serverAddress)"
         homeserverReturnValue = configuration.homeserverURL
         urlForOidcOidcConfigurationPromptLoginHintDeviceIdAdditionalScopesReturnValue = OAuthAuthorizationDataSDKMock(configuration: configuration)
-        loginUsernamePasswordInitialDeviceNameDeviceIdClosure = { username, password, _, _ in
+        loginUsernamePasswordInitialDeviceNameDeviceIdClosure = { [weak self] username, password, _, _ in
             guard username == configuration.validCredentials.username,
                   password == configuration.validCredentials.password else {
                 throw MockError.generic // use the matrix error
             }
+            if username.hasPrefix("@"), username.contains(":") {
+                self?.userIdReturnValue = username
+            } else {
+                self?.userIdReturnValue = "@\(username):\(configuration.serverAddress)"
+            }
         }
         
         userIdReturnValue = configuration.userID