test: common.fixturesDir -> common.fixtures

Update test-https-agent-session-reuse.js

PR-URL: #16028
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Ruben Bridgewater <ruben@bridgewater.de>
Reviewed-By: Gibson Fahnestock <gibfahn@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Michael Dawson <michael_dawson@ca.ibm.com>

test/parallel/test-dgram-send-empty-array.js

@@ -14,7 +14,7 @@ let interval;
 
 client.on('message', common.mustCall(function onMessage(buf, info) {
   const expected = Buffer.alloc(0);
-  assert.ok(buf.equals(expected), 'message was received correctly');
+  assert.ok(buf.equals(expected), `Expected empty message but got ${buf}`);
   clearInterval(interval);
   client.close();
 }));

test/parallel/test-https-agent-session-reuse.js

@@ -7,15 +7,14 @@ if (!common.hasCrypto)
 
 const https = require('https');
 const crypto = require('crypto');
-
-const fs = require('fs');
+const fixtures = require('../common/fixtures');
 
 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')
 };
 
-const ca = fs.readFileSync(`${common.fixturesDir}/keys/ca1-cert.pem`);
+const ca = fixtures.readKey('ca1-cert.pem');
 
 const clientSessions = {};
 let serverRequests = 0;