The test is multi-threaded and expects both threads to receive at least
some of the incoming connections 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.
As mentioned in the change for the udp_reuseport test from a few days
ago, the test is not perfect (there being > 1 core doesn't guarantee
we actually get to run on them) but it's better than nothing.
Fixes: https://github.com/libuv/libuv/issues/5003
