test: updated test-http-get-pipeline-problem.js to use fixtures module

Instead of using common.fixturesDir, uses the fixtures module

Author: NigelKibodeaux

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

@@ -24,6 +24,7 @@
 // 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');
@@ -32,7 +33,7 @@ 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}`);