The test is multi-threaded and expects both threads to receive at least
some of the incoming datagrams but there isn't always true parallelism
under QEMU's user-mode emulator.
Single-core systems are also susceptible to that so also add a check
that we have at least two cores to run on.
It's not perfect because a sufficiently dedicated test torturer could
probably concoct a containerized setup where the core count > 1 but the
available CPU slice is so small that the test effectively still runs
sequentially, but it's better than nothing.
Fixes: https://github.com/libuv/libuv/issues/4777
