Refactor/curp client tests suits

[iGxnon]

Subsequent PR of #584 , add tests

Please briefly answer these questions:

• what problem are you trying to solve? (or if there's no problem, what's the motivation for this change?)

• what changes does this pull request make?

• are there any non-obvious implications of these changes? (does it break compatibility with previous versions, etc)

[codecov]

Codecov Report

Attention: 496 lines in your changes are missing coverage. Please review.

Comparison is base (fa656ad) 64.40% compared to head (dc56e3f) 78.22%.
Report is 43 commits behind head on master.

Files	Patch %	Lines
curp/src/client_new/unary.rs	57.74%	199 Missing ⚠️
curp/src/client_new/mod.rs	6.54%	100 Missing ⚠️
curp/src/rpc/connect.rs	1.31%	75 Missing ⚠️
curp/src/client_new/retry.rs	65.18%	47 Missing ⚠️
curp/src/client_new/tests.rs	93.91%	23 Missing ⚠️
xline/src/server/kv_server.rs	31.25%	22 Missing ⚠️
xline-client/src/clients/kv.rs	78.57%	6 Missing ⚠️
xlineapi/src/command.rs	64.70%	6 Missing ⚠️
curp/src/rpc/mod.rs	94.82%	3 Missing ⚠️
xline/src/server/command.rs	70.00%	3 Missing ⚠️
... and 9 more

Additional details and impacted files

@@             Coverage Diff             @@
##           master     #582       +/-   ##
===========================================
+ Coverage   64.40%   78.22%   +13.82%     
===========================================
  Files         105      158       +53     
  Lines       18802    22438     +3636     
  Branches    18802        0    -18802     
===========================================
+ Hits        12109    17552     +5443     
+ Misses       5955     4886     -1069     
+ Partials      738        0      -738     

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[Phoenix500526]

LGTM

[themanforfree]

LGTM