test: remove setImmediate from timeout test

In test-http2-session-timeout, setImmediate() is used to wrap makeReq().
makeReq() is asynchronous and setImmediate() is not necessary.

Backport-PR-URL: #22850
PR-URL: #23058
Reviewed-By: Ruben Bridgewater <ruben@bridgewater.de>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Refael Ackermann <refack@gmail.com>
Reviewed-By: Richard Lau <riclau@uk.ibm.com>
Reviewed-By: Trivikram Kamat <trivikr.dev@gmail.com>
Reviewed-By: Sakthipriyan Vairamani <thechargingvolcano@gmail.com>

Author: Trott

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

@@ -36,7 +36,7 @@ server.listen(0, common.mustCall(() => {
       const diff = process.hrtime(startTime);
       const milliseconds = (diff[0] * 1e3 + diff[1] / 1e6);
       if (milliseconds < serverTimeout * 2) {
-        setImmediate(makeReq);
+        makeReq();
       } else {
         server.removeListener('timeout', mustNotCall);
         server.close();