test: use of fixtures in test-pipe-head

Replace usage of common.fixturesDir by using common.fixtures
module in test/parallel/test-pipe-head

PR-URL: nodejs/node#15868
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Tobias Nießen <tniessen@tnie.de>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>

test/parallel/test-pipe-head.js

@@ -1,12 +1,12 @@
 'use strict';
 const common = require('../common');
+const fixtures = require('../common/fixtures');
 const assert = require('assert');
 
 const exec = require('child_process').exec;
-const join = require('path').join;
 
 const nodePath = process.argv[0];
-const script = join(common.fixturesDir, 'print-10-lines.js');
+const script = fixtures.path('print-10-lines.js');
 
 const cmd = `"${nodePath}" "${script}" | head -2`;