test: improve reliability in http2-session-timeout

Use `setImmediate()` instead of `setTimeout()` to improve robustness of
test-http2-session-timeout.

Fixes: #20628

PR-URL: #22026
Reviewed-By: Anatoli Papirovski <apapirovski@mac.com>
Reviewed-By: Jon Moss <me@jonathanmoss.me>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Trivikram Kamat <trivikr.dev@gmail.com>

Author: Trott

test/sequential/test-http2-session-timeout.js

@@ -3,13 +3,12 @@
 const common = require('../common');
 if (!common.hasCrypto)
   common.skip('missing crypto');
-const h2 = require('http2');
+const http2 = require('http2');
 
 const serverTimeout = common.platformTimeout(200);
-const callTimeout = common.platformTimeout(20);
 const mustNotCall = common.mustNotCall();
 
-const server = h2.createServer();
+const server = http2.createServer();
 server.timeout = serverTimeout;
 
 server.on('request', (req, res) => res.end());
@@ -19,7 +18,7 @@ server.listen(0, common.mustCall(() => {
   const port = server.address().port;
 
   const url = `http://localhost:${port}`;
-  const client = h2.connect(url);
+  const client = http2.connect(url);
   const startTime = process.hrtime();
   makeReq();
 
@@ -37,7 +36,7 @@ server.listen(0, common.mustCall(() => {
       const diff = process.hrtime(startTime);
       const milliseconds = (diff[0] * 1e3 + diff[1] / 1e6);
       if (milliseconds < serverTimeout * 2) {
-        setTimeout(makeReq, callTimeout);
+        setImmediate(makeReq);
       } else {
         server.removeListener('timeout', mustNotCall);
         server.close();