Configure the AuthenticationService later now that we have 2 flows on the start screen. (#3316)

* Don't query the homeserver until confirming it (or selecting a different one).

* Setup the infrastructure to test AuthenticationService.

Implement basic tests for configuration & password login.

* Use the real AuthenticationService with a mock Client in all of the tests.

* Add tests for the ServerConfirmationScreenViewModel.

* Remove redundant view state and test for it.

ElementX/Sources/Mocks/UserSessionStoreMock.swift

@@ -0,0 +1,17 @@
+//
+// Copyright 2024 New Vector Ltd.
+//
+// SPDX-License-Identifier: AGPL-3.0-only
+// Please see LICENSE in the repository root for full details.
+//
+
+extension UserSessionStoreMock {
+    struct Configuration { }
+    
+    convenience init(configuration: Configuration) {
+        self.init()
+        
+        userSessionForSessionDirectoriesPassphraseReturnValue = .success(UserSessionMock(.init(clientProxy: ClientProxyMock(.init()))))
+        clientSessionDelegate = KeychainControllerMock()
+    }
+}