test/librados/aio_cxx: skip EIO boundary assertion when no in-flight I/Os

test/librados/aio_cxx: skip EIO boundary assertion when no in-flight I/Os

In the PoolEIOFlag test we now track whether any async writes were still
in flight at the moment we flipped the pool’s `eio=true` flag using an
atomic `missed_eio` flag set inside the helper thread.  If we never
actually overlapped any in-flight AIOs (we completed all aio successfully)
with the flag flip, we `GTEST_SKIP()` the final `max_success+1 == min_failed`
boundary assertion, avoiding spurious failures when all writes complete
before EIO takes effect.

Fixes: https://tracker.ceph.com/issues/70852
Signed-off-by: Nitzan Mordechai <nmordech@redhat.com>