test: make http2 timeout test robust

Instead of using magic values for the server timeout in
test-http2-session-timeout, measure the duration of the first request
(which should be longer than subsequent requests) and use that value.

Fixes: #20628

test/sequential/sequential.status

@@ -23,5 +23,3 @@ test-http2-large-file: PASS, FLAKY
 [$system==aix]
 
 [$arch==arm]
-# https://github.com/nodejs/node/issues/20628
-test-http2-session-timeout: PASS,FLAKY

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

@@ -6,15 +6,15 @@ if (!common.hasCrypto)
 const assert = require('assert');
 const http2 = require('http2');
 
-const serverTimeout = common.platformTimeout(200);
-
 let requests = 0;
 const mustNotCall = () => {
   assert.fail(`Timeout after ${requests} request(s)`);
 };
 
 const server = http2.createServer();
-server.timeout = serverTimeout;
+// Disable server timeout until first request. We will set the timeout based on
+// how long the first request takes.
+server.timeout = 0;
 
 server.on('request', (req, res) => res.end());
 server.on('timeout', mustNotCall);
@@ -24,7 +24,7 @@ server.listen(0, common.mustCall(() => {
 
   const url = `http://localhost:${port}`;
   const client = http2.connect(url);
-  const startTime = process.hrtime();
+  let startTime = process.hrtime();
   makeReq();
 
   function makeReq() {
@@ -42,7 +42,15 @@ server.listen(0, common.mustCall(() => {
     request.on('end', () => {
       const diff = process.hrtime(startTime);
       const milliseconds = (diff[0] * 1e3 + diff[1] / 1e6);
-      if (milliseconds < serverTimeout * 2) {
+      if (server.timeout === 0) {
+        // Set the timeout now. First connection will take significantly longer
+        // than subsequent connections, so using the duration of the first
+        // connection as the timeout should be robust. Double it anyway for good
+        // measure.
+        server.timeout = milliseconds * 2;
+        startTime = process.hrtime();
+        makeReq();
+      } else if (milliseconds < server.timeout * 2) {
         makeReq();
       } else {
         server.removeListener('timeout', mustNotCall);