UpcallTestHelper::runInNewProcess waits for forked process without timeout

UpcallTestHelper::runInNewProcess creates forked processes which are used to run upcall related tests.

We saw a very rare deadclock issue when running this test, and investigation shows that one of the forked processes is stuck either when starting or shutting down the JVM (not responding to jcmd).

We should add a timeout here, so that deadlocked processes actually terminate.