test: fix pummel test failures

A handful of tests in `test/pummel` were failing due to undefined
variables.

The tests in pummel are not run in CI or otherwise exercised regularly
so these failures can go unnoticed for a long time.

test/pummel/test-crypto-dh.js

@@ -1,6 +1,7 @@
 'use strict';
-var common = require('../common');
-var assert = require('assert');
+const common = require('../common');
+const assert = require('assert');
+const crypto = require('crypto');
 
 if (!common.hasCrypto) {
   console.log('1..0 # Skipped: node compiled without OpenSSL.');

test/pummel/test-dtrace-jsstack.js

@@ -14,7 +14,7 @@ if (os.type() != 'SunOS') {
 var frames = [ 'stalloogle', 'bagnoogle', 'doogle' ];
 
 var stalloogle = function(str) {
-  expected = str;
+  global.expected = str;
   os.loadavg();
 };
 

test/pummel/test-net-throttle.js

@@ -1,15 +1,15 @@
 'use strict';
-var common = require('../common');
-var assert = require('assert');
-var net = require('net');
+const common = require('../common');
+const assert = require('assert');
+const net = require('net');
 
-var N = 1024 * 1024;
-var part_N = N / 3;
+const N = 1024 * 1024;
+const part_N = N / 3;
 var chars_recved = 0;
 var npauses = 0;
 
 console.log('build big string');
-body = 'C'.repeat(N);
+const body = 'C'.repeat(N);
 
 console.log('start server on port ' + common.PORT);