test: use fixtures.readKey

Replace the common.fixturesDir by fixtures.readKey
in http2 subsystem test 'server startup'.

PR-URL: #15892
Reviewed-By: Sam Roberts <vieuxtech@gmail.com>
Reviewed-By: Daijiro Wachi <daijiro.wachi@gmail.com>
Reviewed-By: Ruben Bridgewater <ruben@bridgewater.de>

test/parallel/test-http2-server-startup.js

@@ -6,22 +6,19 @@
 // other than start listening.
 
 const common = require('../common');
+const commonFixtures = require('../common/fixtures');
 
 if (!common.hasCrypto)
   common.skip('missing crypto');
 
 const assert = require('assert');
 const http2 = require('http2');
-const path = require('path');
 const tls = require('tls');
 const net = require('net');
-const fs = require('fs');
 
 const options = {
-  key: fs.readFileSync(
-    path.resolve(common.fixturesDir, 'keys/agent2-key.pem')),
-  cert: fs.readFileSync(
-    path.resolve(common.fixturesDir, 'keys/agent2-cert.pem'))
+  key: commonFixtures.readKey('agent2-key.pem'),
+  cert: commonFixtures.readKey('agent2-cert.pem')
 };
 
 // There should not be any throws