test: use fixtures module in test

Instead of using common.fixturesDir, uses the fixtures module in
test-http-get-pipeline-problem.

PR-URL: #16117
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Daijiro Wachi <daijiro.wachi@gmail.com>
Reviewed-By: Rich Trott <rtrott@gmail.com>

test/parallel/test-http-get-pipeline-problem.js

@@ -3,6 +3,7 @@
 // after http.globalAgent.maxSockets number of files.
 // See https://groups.google.com/forum/#!topic/nodejs-dev/V5fB69hFa9o
 const common = require('../common');
+const fixtures = require('../common/fixtures');
 const assert = require('assert');
 const http = require('http');
 const fs = require('fs');
@@ -11,7 +12,7 @@ http.globalAgent.maxSockets = 1;
 
 common.refreshTmpDir();
 
-const image = fs.readFileSync(`${common.fixturesDir}/person.jpg`);
+const image = fixtures.readSync('/person.jpg');
 
 console.log(`image.length = ${image.length}`);