Fix occasionally ConcurrentModificationException in Netty tests

Due to using stream() on a log appender while the appender is still running and possibly adding more logs.

java/memory/memory-netty/src/test/java/org/apache/arrow/memory/netty/TestNettyAllocator.java

@@ -19,6 +19,7 @@
 
 import static org.junit.Assert.assertTrue;
 
+import java.util.Collections;
 import java.util.stream.Collectors;
 
 import org.apache.arrow.memory.ArrowBuf;
@@ -40,6 +41,7 @@ public class TestNettyAllocator {
   @Test
   public void testMemoryUsage() {
     ListAppender<ILoggingEvent> memoryLogsAppender = new ListAppender<>();
+    memoryLogsAppender.list = Collections.synchronizedList(memoryLogsAppender.list);
     Logger logger = (Logger) LoggerFactory.getLogger("arrow.allocator");
     try {
       logger.setLevel(Level.TRACE);
@@ -52,13 +54,17 @@ public void testMemoryUsage() {
       boolean result = false;
       long startTime = System.currentTimeMillis();
       while ((System.currentTimeMillis() - startTime) < 10000) { // 10 seconds maximum for time to read logs
-        result = memoryLogsAppender.list.stream()
-            .anyMatch(
-                log -> log.toString().contains("Memory Usage: \n") &&
-                    log.toString().contains("Large buffers outstanding: ") &&
-                    log.toString().contains("Normal buffers outstanding: ") &&
-                    log.getLevel().equals(Level.TRACE)
-            );
+        // Lock on the list backing the appender since a background thread might try to add more logs
+        // while stream() is iterating over list elements. This would throw a flakey ConcurrentModificationException.
+        synchronized (memoryLogsAppender.list) {
+          result = memoryLogsAppender.list.stream()
+              .anyMatch(
+                  log -> log.toString().contains("Memory Usage: \n") &&
+                      log.toString().contains("Large buffers outstanding: ") &&
+                      log.toString().contains("Normal buffers outstanding: ") &&
+                      log.getLevel().equals(Level.TRACE)
+              );
+        }
         if (result) {
           break;
         }