test: replace common.fixturesDir with fixtures.readSync()

Replace common.fixturesDir with fixtures.readSync() in
test-tls-getprotocol.

PR-URL: #16802
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Gireesh Punathil <gpunathi@in.ibm.com>
Reviewed-By: Rich Trott <rtrott@gmail.com>

test/parallel/test-tls-getprotocol.js

@@ -3,9 +3,12 @@ const common = require('../common');
 if (!common.hasCrypto)
   common.skip('missing crypto');
 
+// This test ensures that `getProtocol` returns the right protocol
+// from a TLS connection
+
 const assert = require('assert');
 const tls = require('tls');
-const fs = require('fs');
+const fixtures = require('../common/fixtures');
 
 const clientConfigs = [
   { secureProtocol: 'TLSv1_method', version: 'TLSv1' },
@@ -14,8 +17,8 @@ const clientConfigs = [
 ];
 
 const serverConfig = {
-  key: fs.readFileSync(`${common.fixturesDir}/keys/agent2-key.pem`),
-  cert: fs.readFileSync(`${common.fixturesDir}/keys/agent2-cert.pem`)
+  key: fixtures.readSync('/keys/agent2-key.pem'),
+  cert: fixtures.readSync('/keys/agent2-cert.pem')
 };
 
 const server = tls.createServer(serverConfig, common.mustCall(function() {