Fix itests on machines with lots of processors (#17263)

Fix itests on machines with lots of processors (#17263)

When this JVM variable is not set Jetty creates selectors based on the number of available processors (`Runtime.getRuntime().availableProcessors()`).
Jetty will also throw an exception when there is a mismatch between the max thread pool size and the number of selectors.
This causes issues with the default thread pool size of 10 on machines with more than 10 available processors.

See also [`SelectorManager.defaultSelectors(Executor)`](https://github.com/jetty/jetty.project/blob/70015831e5195ba74da6383c4ab7c90c56bb08af/jetty-io/src/main/java/org/eclipse/jetty/io/SelectorManager.java#L71-L80).

Signed-off-by: Wouter Born <github@maindrain.net>