test: replace fixtureDir with fixtures module

This commit replaces the usage of common.fixturesDir with
fixtures.path.

PR-URL: #15823
Reviewed-By: Ryan Graham <r.m.graham@gmail.com>
Reviewed-By: Benjamin Gruenbaum <benjamingr@gmail.com>
Reviewed-By: Richard Lau <riclau@uk.ibm.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Tobias Nießen <tniessen@tnie.de>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>

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

@@ -4,9 +4,11 @@
 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 fs = require('fs');
 
 const {
@@ -15,7 +17,7 @@ const {
   HTTP2_HEADER_LAST_MODIFIED
 } = http2.constants;
 
-const fname = path.resolve(common.fixturesDir, 'elipses.txt');
+const fname = fixtures.path('elipses.txt');
 const data = fs.readFileSync(fname);
 const stat = fs.statSync(fname);
 const fd = fs.openSync(fname, 'r');