[ml] Register prefix change listener with test root disposable

To make sure it gets disposed after the test is done executing.

GitOrigin-RevId: a7add689a01f6d175632a1cba6200b67dc41256e

plugins/completion-ml-ranking/test/com/intellij/completion/ml/tracker/PrefixChangeListenerTest.kt

@@ -1,18 +1,4 @@
-/*
- * Copyright 2000-2017 JetBrains s.r.o.
- *
- * Licensed under the Apache License, Version 2.0 (the "License");
- * you may not use this file except in compliance with the License.
- * You may obtain a copy of the License at
- *
- * http://www.apache.org/licenses/LICENSE-2.0
- *
- * Unless required by applicable law or agreed to in writing, software
- * distributed under the License is distributed on an "AS IS" BASIS,
- * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
- * See the License for the specific language governing permissions and
- * limitations under the License.
- */
+// Copyright 2000-2025 JetBrains s.r.o. and contributors. Use of this source code is governed by the Apache 2.0 license.
 package com.intellij.completion.ml.tracker
 
 import com.intellij.codeInsight.completion.LightFixtureCompletionTestCase
@@ -37,7 +23,7 @@ class PrefixChangeListenerTest: LightFixtureCompletionTestCase() {
   fun `test prefix change listener`() {
     myFixture.completeBasic()
 
-    lookup.setPrefixChangeListener(object : PrefixChangeListener {
+    lookup.addPrefixChangeListener(object : PrefixChangeListener {
       override fun afterAppend(c: Char) {
         afterChange.clear()
         afterChange.addAll(lookup.items)
@@ -57,7 +43,7 @@ class PrefixChangeListenerTest: LightFixtureCompletionTestCase() {
         beforeChange.clear()
         beforeChange.addAll(lookup.items)
       }
-    })
+    }, testRootDisposable)
 
     lastLookupState.clear()
     lastLookupState.addAll(lookup.items)