test: increase the test code coverage a little bit

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 40.90909% with 13 lines in your changes missing coverage. Please review.

Project coverage is 81.03%. Comparing base (36df9ef) to head (fa08d03).

Files	Patch %	Lines
acceptor_unix.go	37.50%	10 Missing :warning:
client_unix.go	50.00%	0 Missing and 3 partials :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## dev #618 +/- ## ========================================== + Coverage 79.43% 81.03% +1.59% ========================================== Files 21 21 Lines 2106 2109 +3 ========================================== + Hits 1673 1709 +36 + Misses 300 271 -29 + Partials 133 129 -4 ``` | [Flag](https://app.codecov.io/gh/panjf2000/gnet/pull/618/flags?src=pr&el=flags&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Andy+Pan) | Coverage Δ | | |---|---|---| | [unittests](https://app.codecov.io/gh/panjf2000/gnet/pull/618/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Andy+Pan) | `81.03% <40.90%> (+1.59%)` | :arrow_up: | Flags with carried forward coverage won't be shown. [Click here](https://docs.codecov.io/docs/carryforward-flags?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Andy+Pan#carryforward-flags-in-the-pull-request-comment) to find out more.

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