test: increase the test code coverage a little bit

[panjf2000]

No description provided.

[codecov]

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 ⚠️
client_unix.go	50.00%	0 Missing and 3 partials ⚠️

Additional details and impacted files

@@            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	Coverage Δ
unittests	81.03% <40.90%> (+1.59%)	⬆️

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