test: update pummel/test-exec

ChildProcess.prototype._internal is called ChildProcess.prototype._handle now.

test/pummel/test-exec.js

@@ -78,7 +78,7 @@ process.nextTick(function() {
   console.log('kill pid %d', killMeTwice.pid);
   // make sure there is no race condition in starting the process
   // the PID SHOULD exist directly following the exec() call.
-  assert.equal('number', typeof killMeTwice._internal.pid);
+  assert.equal('number', typeof killMeTwice._handle.pid);
   // Kill the process
   killMeTwice.kill();
 });