test: add mock test for pouch package client

[allencloud]

Signed-off-by: Allen Sun allensun.shl@alibaba-inc.com

Ⅰ. Describe what this PR did

For more reliable client package which could be a golang SDK for Pouch developer, we should add more guarantee work on this. This PR adds mock test for the client package.

In the code, we add a mock server in the client when unit testing.

Ⅱ. Does this pull request fix one issue?

none

Ⅲ. Describe how you did it

none

Ⅳ. Describe how to verify it

none

Ⅴ. Special notes for reviews

none

[codecov-io]

Codecov Report

Merging #965 into master will increase coverage by 0.56%.
The diff coverage is 71.42%.

@@            Coverage Diff            @@
##           master    #965      +/-   ##
=========================================
+ Coverage   12.94%   13.5%   +0.56%     
=========================================
  Files         123     124       +1     
  Lines        8314    8314              
=========================================
+ Hits         1076    1123      +47     
+ Misses       7142    7088      -54     
- Partials       96     103       +7

Impacted Files	Coverage Δ
client/container.go	0% <ø> (ø)	⬆️
client/container_start.go	71.42% <71.42%> (ø)
client/request.go	46.34% <0%> (+46.34%)	⬆️
client/utils.go	50% <0%> (+50%)	⬆️

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[Letty5411]

LGTM