test/epwait: change the epwait.t:test/epwait to count SQEs/CQEs and check for sqe->user_data/cqe->user_data

[schlad]

test_race() relied on a fixed loop count and implicit CQ draining to
validate EPOLL_WAIT behavior under concurrent wakeups. This lead to
hangs in the VMs with lower number of CPUs. This patch reworks the test,
so it explicitly counts SQEs submitted and CQEs completed. The writer
thread is now driven by an atomic run flag instead of a fixed loop
count, ensuring deterministic shutdown and stable completion accounting.

test_race() relied on a fixed loop count and implicit CQ draining to
validate EPOLL_WAIT behavior under concurrent wakeups. This lead to
hangs in the VMs with lower number of CPUs. This patch reworks the test,
so it explicitly counts SQEs submitted and CQEs completed. The writer
thread is now driven by an atomic run flag instead of a fixed loop
count, ensuring deterministic shutdown and stable completion accounting.