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

@@ -52,13 +52,17 @@ public void testMemoryUsage() {
       boolean result = false;
       long startTime = System.currentTimeMillis();
       while ((System.currentTimeMillis() - startTime) < 10000) { // 10 seconds maximum for time to read logs
+        // Pause the appender while collecting results to avoid concurrent modification exceptions
+        // since another thread might be using the logger.
+        memoryLogsAppender.stop();
         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)
             );
+        memoryLogsAppender.start();
         if (result) {
           break;
         }