fix "io_getevents returns -4" problem

Appears for many times in unit test:
```
W2018-07-18 10:16:05.669 (1531880165669313326 617f) mimic.io-thrd.24959: io_getevents returns -4, you probably want to try on another machine:-(
```

Start by reproducing the repeated `io_getevents returns -4` message in the unit test and trace the `mimic.io-thrd` path that emits it. Investigate the failing event calls and their environment assumptions; done means the unit test no longer repeatedly reports this error, or the issue clearly documents a confirmed machine or platform limitation.

Written by the indexing model from the issue text.