test: remove unused util imports

A number of tests in `test/parallel` were importing the `util` module
via `require()` but not using `util` for anything. This removes those
`require()` statements.

PR-URL: #4562
Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>

Author: Trott

test/parallel/test-beforeexit-event.js

@@ -1,7 +1,6 @@
 'use strict';
 var assert = require('assert');
 var net = require('net');
-var util = require('util');
 var common = require('../common');
 var revivals = 0;
 var deaths = 0;

test/parallel/test-child-process-fork-regr-gh-2847.js

@@ -5,7 +5,6 @@ const assert = require('assert');
 
 const cluster = require('cluster');
 const net = require('net');
-const util = require('util');
 
 var connectcount = 0;
 var sendcount = 0;

test/parallel/test-cluster-worker-isconnected.js

@@ -2,7 +2,6 @@
 require('../common');
 var cluster = require('cluster');
 var assert = require('assert');
-var util = require('util');
 
 if (cluster.isMaster) {
   var worker = cluster.fork();

test/parallel/test-file-write-stream2.js

@@ -4,7 +4,6 @@ var assert = require('assert');
 
 var path = require('path');
 var fs = require('fs');
-var util = require('util');
 
 
 var filepath = path.join(common.tmpDir, 'write.txt');

test/parallel/test-file-write-stream3.js

@@ -3,8 +3,7 @@ var common = require('../common');
 var assert = require('assert');
 
 var path = require('path'),
-    fs = require('fs'),
-    util = require('util');
+    fs = require('fs');
 
 
 var filepath = path.join(common.tmpDir, 'write_pos.txt');

test/parallel/test-http-buffer-sanity.js

@@ -2,7 +2,6 @@
 var common = require('../common');
 var assert = require('assert');
 var http = require('http');
-var util = require('util');
 
 var bufferSize = 5 * 1024 * 1024;
 var measuredSize = 0;

test/parallel/test-http-head-request.js

@@ -2,7 +2,6 @@
 var common = require('../common');
 var assert = require('assert');
 var http = require('http');
-var util = require('util');
 
 
 var body = 'hello world\n';

test/parallel/test-http-keep-alive-close-on-header.js

@@ -2,7 +2,6 @@
 var common = require('../common');
 var assert = require('assert');
 var http = require('http');
-var util = require('util');
 
 var body = 'hello world\n';
 var headers = {'connection': 'keep-alive'};

test/parallel/test-pipe-return-val.js

@@ -4,7 +4,6 @@
 require('../common');
 var Stream = require('stream').Stream;
 var assert = require('assert');
-var util = require('util');
 
 var sourceStream = new Stream();
 var destStream = new Stream();

test/parallel/test-repl-persistent-history.js

@@ -7,7 +7,6 @@ const stream = require('stream');
 const REPL = require('internal/repl');
 const assert = require('assert');
 const fs = require('fs');
-const util = require('util');
 const path = require('path');
 const os = require('os');