Fix the bug of epoll: EEXIST error when two different files with the same fd about ngo HOT 1 CLOSED

This is a known limitation (at least known to me 😂 ) of the current epoll implementation. The epoll implementation in the mainstream Occlum has a fix to this problem. But the solution is not very elegant. So I didn't adopt it in NGO. As a workaround, let's manually epoll delete a fd before closing it in unit tests.

Eventually, we need to automatically delete a fd from all its associated epoll file when a fd is closed. This is a high priority issue since we cannot expect user programs to have the courtesy of epoll deleting every fd.

from ngo.