test: replace common.fixturesDir with fixture

PR-URL: #15940
Reviewed-By: Sam Roberts <vieuxtech@gmail.com>
Reviewed-By: Ruben Bridgewater <ruben@bridgewater.de>
Reviewed-By: Tobias Nießen <tniessen@tnie.de>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>

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

@@ -5,9 +5,9 @@ const common = require('../common');
 if (!common.hasCrypto)
  common.skip('missing crypto');
 const http2 = require('http2');
-const path = require('path');
+const fixtures = require('../common/fixtures');
 
-const fname = path.resolve(common.fixturesDir, 'elipses.txt');
+const fname = fixtures.path('elipses.txt');
 
 const server = http2.createServer(common.mustCall((request, response) => {
  response.stream.respondWithFile(fname);