tests: eventfd: test wake-up from pending poll()

Make sure that `poll(..., POLLIN)` executed from another thread returns
when `eventfd_write()` is called. Test also the same with `poll(...,
POLLOUT)` and `eventfd_read()` on eventfd with counter equal to
`UINT64_MAX - 1`.

Signed-off-by: Marcin Niestroj <m.niestroj@emb.dev>