test: add common.fixtures to https-req-split

Replaced readFileSync with fixtures.readKey.

PR-URL: nodejs/node#15801
Reviewed-By: Richard Lau <riclau@uk.ibm.com>
Reviewed-By: Daijiro Wachi <daijiro.wachi@gmail.com>
Reviewed-By: Gireesh Punathil <gpunathi@in.ibm.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Benjamin Gruenbaum <benjamingr@gmail.com>
Reviewed-By: Rich Trott <rtrott@gmail.com>

test/parallel/test-https-req-split.js

@@ -21,6 +21,7 @@
 
 'use strict';
 const common = require('../common');
+const fixtures = require('../common/fixtures');
 if (!common.hasCrypto)
   common.skip('missing crypto');
 
@@ -30,11 +31,10 @@ process.env.NODE_TLS_REJECT_UNAUTHORIZED = '0';
 const https = require('https');
 
 const tls = require('tls');
-const fs = require('fs');
 
 const options = {
-  key: fs.readFileSync(`${common.fixturesDir}/keys/agent1-key.pem`),
-  cert: fs.readFileSync(`${common.fixturesDir}/keys/agent1-cert.pem`)
+  key: fixtures.readKey('agent1-key.pem'),
+  cert: fixtures.readKey('agent1-cert.pem')
 };
 
 // Force splitting incoming data