http2: replace fixturesDir with common.fixtures

PR-URL: nodejs/node#15839
Reviewed-By: Ryan Graham <r.m.graham@gmail.com>
Reviewed-By: Gibson Fahnestock <gibfahn@gmail.com>
Reviewed-By: Benjamin Gruenbaum <benjamingr@gmail.com>
Reviewed-By: Gireesh Punathil <gpunathi@in.ibm.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>

test/parallel/test-http2-respond-file-304.js

@@ -3,16 +3,16 @@
 const common = require('../common');
 if (!common.hasCrypto)
   common.skip('missing crypto');
+const fixtures = require('../common/fixtures');
 const http2 = require('http2');
 const assert = require('assert');
-const path = require('path');
 
 const {
   HTTP2_HEADER_CONTENT_TYPE,
   HTTP2_HEADER_STATUS
 } = http2.constants;
 
-const fname = path.resolve(common.fixturesDir, 'elipses.txt');
+const fname = fixtures.path('elipses.txt');
 
 const server = http2.createServer();
 server.on('stream', (stream) => {