feat: automatically SetMode to TestMode when run go test.

[micanzhang]

related issue: #3134

[micanzhang]

@appleboy PTAL

[codecov]

Codecov Report

Merging #3139 (7f014d9) into master (90e7073) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master    #3139   +/-   ##
=======================================
  Coverage   98.77%   98.77%           
=======================================
  Files          41       41           
  Lines        3109     3112    +3     
=======================================
+ Hits         3071     3074    +3     
  Misses         26       26           
  Partials       12       12           

Flag	Coverage Δ
go-1.14	98.77% <100.00%> (+<0.01%)	⬆️
go-1.15	98.61% <100.00%> (+<0.01%)	⬆️
go-1.16	98.59% <100.00%> (+<0.01%)	⬆️
go-1.17	98.52% <100.00%> (+<0.01%)	⬆️
go-1.18	98.52% <100.00%> (+<0.01%)	⬆️
macos-latest	98.61% <100.00%> (-0.16%)	⬇️
nomsgpack	98.76% <100.00%> (+<0.01%)	⬆️
ubuntu-latest	98.77% <100.00%> (+<0.01%)	⬆️

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Impacted Files	Coverage Δ
mode.go	100.00% <100.00%> (ø)

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[micanzhang]

@appleboy PTAL

[appleboy]

Need @thinkerou approval.