Constrain available processors to Jetty during test runs to prevent thread exhaustion.

As described during a release candidate vote:

HIVE-24484 introduced a change to limit hive.server2.webui.max.threads to 4. Jetty enforces thread leasing to warn or abort if there aren't enough threads available [1]. During startup, it attempts to lease a thread per NIO selector [2]. By default, the number of NIO selectors to use is determined based on available CPUs [3]. This is mostly a passthrough to Runtime.availableProcessors() [4]. In my case, running on a machine with 16 CPUs, this ended up creating more than 4 selectors, therefore requiring more than 4 threads and violating the lease check. I was able to work around this by passing the JETTY_AVAILABLE_PROCESSORS system property to constrain the number of CPUs available to Jetty.

Since we are intentionally constraining the pool to 4 threads during itests, let's also limit JETTY_AVAILABLE_PROCESSORS in maven.test.jvm.args of the root pom.xml, so that others don't run into this problem later.

[1] https://github.com/eclipse/jetty.project/blob/jetty-9.4.40.v20210413/jetty-util/src/main/java/org/eclipse/jetty/util/thread/ThreadPoolBudget.java#L165
[2] https://github.com/eclipse/jetty.project/blob/jetty-9.4.40.v20210413/jetty-io/src/main/java/org/eclipse/jetty/io/SelectorManager.java#L255
[3] https://github.com/eclipse/jetty.project/blob/jetty-9.4.40.v20210413/jetty-io/src/main/java/org/eclipse/jetty/io/SelectorManager.java#L79
[4] https://github.com/eclipse/jetty.project/blob/jetty-9.4.40.v20210413/jetty-util/src/main/java/org/eclipse/jetty/util/ProcessorUtils.java#L45