test(route): expose performRequest func

[appleboy]

There are many packages that need to testing gin router like as below:

https://github.com/gin-contrib/static/blob/0b63c38653d9941a7851807d76e2692655cf6fc7/static_test.go#L16-L21

So we can expose the performRequest function.

[codecov]

Codecov Report

Merging #3012 (5d82745) into master (0136319) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #3012   +/-   ##
=======================================
  Coverage   98.60%   98.60%           
=======================================
  Files          41       41           
  Lines        3084     3084           
=======================================
  Hits         3041     3041           
  Misses         30       30           
  Partials       13       13           

Flag	Coverage Δ
go-1.13	∅ <ø> (∅)
go-1.14	∅ <ø> (∅)
go-1.15	∅ <ø> (∅)
go-1.16	98.60% <ø> (ø)
go-1.17	98.50% <ø> (ø)
macos-latest	98.60% <ø> (ø)
nomsgpack	98.58% <ø> (ø)
ubuntu-latest	98.60% <ø> (ø)

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[thinkerou]

lgtm