test: upgrade from fixturesDir to fixtures.path

The common/fixtures module provides convenience methods
for working with files in the test/fixtures directory.

PR-URL: #15960
Reviewed-By: Daijiro Wachi <daijiro.wachi@gmail.com>
Reviewed-By: Benjamin Gruenbaum <benjamingr@gmail.com>
Reviewed-By: Ruben Bridgewater <ruben@bridgewater.de>
Reviewed-By: Tobias Nießen <tniessen@tnie.de>

test/parallel/test-http2-respond-file-204.js

@@ -4,16 +4,16 @@
 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) => {