allow reflection of MXBean for file descriptor stats

dev-tools/tests.policy

@@ -81,6 +81,8 @@ grant {
   permission java.lang.RuntimePermission "accessClassInPackage.sun.nio.ch";
   // needed by groovy engine
   permission java.lang.RuntimePermission "accessClassInPackage.sun.reflect";
+  // needed to get file descriptor statistics
+  permission java.lang.RuntimePermission "accessClassInPackage.sun.management";
 
   permission java.lang.RuntimePermission "accessDeclaredMembers";
   permission java.lang.RuntimePermission "getStackTrace";