LUCENE-10118: Test fix

We need to collect messages in a thread-safe list, as we're writing from multiple
threads.

lucene/core/src/test/org/apache/lucene/index/TestConcurrentMergeScheduler.java

@@ -19,6 +19,7 @@ package org.apache.lucene.index;
 import com.carrotsearch.randomizedtesting.generators.RandomStrings;
 import java.io.IOException;
 import java.util.ArrayList;
+import java.util.Collections;
 import java.util.List;
 import java.util.Map;
 import java.util.Set;
@@ -638,7 +639,7 @@ public class TestConcurrentMergeScheduler extends LuceneTestCase {
         };
     iwc.setMergeScheduler(cms);
 
-    List<String> messages = new ArrayList<>();
+    List<String> messages = Collections.synchronizedList(new ArrayList<>());
     iwc.setInfoStream(
         new InfoStream() {
           @Override
@@ -682,7 +683,12 @@ public class TestConcurrentMergeScheduler extends LuceneTestCase {
           messages.stream()
               .filter(line -> line.startsWith("merge thread " + name))
               .collect(Collectors.toList());
-      assertTrue(threadMsgs.size() >= 3);
+      assertTrue(
+          "Expected:·a·value·equal·to·or·greater·than·3,·got:"
+              + threadMsgs.size()
+              + ", threadMsgs="
+              + threadMsgs,
+          threadMsgs.size() >= 3);
       assertTrue(threadMsgs.get(0).startsWith("merge thread " + name + " start"));
       assertTrue(
           threadMsgs.stream()