test: use fixtures module in test-https-truncate

PR-URL: #15875
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Yuta Hiroto <hello@about-hiroppy.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Ruben Bridgewater <ruben@bridgewater.de>

test/parallel/test-https-truncate.js

@@ -26,12 +26,11 @@ if (!common.hasCrypto)
   common.skip('missing crypto');
 
 const assert = require('assert');
+const fixtures = require('../common/fixtures');
 const https = require('https');
 
-const fs = require('fs');
-
-const key = fs.readFileSync(`${common.fixturesDir}/keys/agent1-key.pem`);
-const cert = fs.readFileSync(`${common.fixturesDir}/keys/agent1-cert.pem`);
+const key = fixtures.readKey('agent1-key.pem');
+const cert = fixtures.readKey('agent1-cert.pem');
 
 // number of bytes discovered empirically to trigger the bug
 const data = Buffer.alloc(1024 * 32 + 1);