8249543: Force DirectBufferAllocTest to run with -ExplicitGCInvokesConcurrent

Reviewed-by: alanb
diff --git a/test/jdk/java/nio/Buffer/DirectBufferAllocTest.java b/test/jdk/java/nio/Buffer/DirectBufferAllocTest.java
index 3a4d815..ab22b69 100644
--- a/test/jdk/java/nio/Buffer/DirectBufferAllocTest.java
+++ b/test/jdk/java/nio/Buffer/DirectBufferAllocTest.java
@@ -26,7 +26,7 @@
  * @bug 6857566
  * @summary DirectByteBuffer garbage creation can outpace reclamation
  *
- * @run main/othervm -XX:MaxDirectMemorySize=128m DirectBufferAllocTest
+ * @run main/othervm -XX:MaxDirectMemorySize=128m -XX:-ExplicitGCInvokesConcurrent DirectBufferAllocTest
  */
 
 import java.nio.ByteBuffer;