Move NotUsedPresenter to the testutils module

The fake presenter was declared in RootFlowNodeTest, so other tests could
not use it. Move it to the testutils module, in a new presenter package,
along with MutablePresenter.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

tests/testutils/src/main/kotlin/io/element/android/tests/testutils/presenter/MutablePresenter.kt

@@ -6,7 +6,7 @@
  * Please see LICENSE files in the repository root for full details.
  */
 
-package io.element.android.tests.testutils
+package io.element.android.tests.testutils.presenter
 
 import androidx.compose.runtime.Composable
 import androidx.compose.runtime.collectAsState

tests/testutils/src/main/kotlin/io/element/android/tests/testutils/presenter/NotUsedPresenter.kt

@@ -0,0 +1,19 @@
+/*
+ * Copyright (c) 2026 Element Creations Ltd.
+ *
+ * SPDX-License-Identifier: AGPL-3.0-only OR LicenseRef-Element-Commercial.
+ * Please see LICENSE files in the repository root for full details.
+ */
+
+package io.element.android.tests.testutils.presenter
+
+import androidx.compose.runtime.Composable
+import io.element.android.libraries.architecture.Presenter
+
+/**
+ * A presenter that cannot be used, util for testing Nodes.
+ */
+class NotUsedPresenter<T> : Presenter<T> {
+    @Composable
+    override fun present(): T = error("Not used")
+}