Fixed the gNMI JSON payload matching issue in testkit.

[boleifu]

This issue causes JSON payload validationg fails when matching JSON
arrays.

[codecov-io]

Codecov Report

Merging #84 into master will decrease coverage by 2.89%.
The diff coverage is 71.42%.

@@            Coverage Diff            @@
##           master      #84     +/-   ##
=========================================
- Coverage   59.48%   56.58%   -2.9%     
=========================================
  Files          14       15      +1     
  Lines        1049     1154    +105     
=========================================
+ Hits          624      653     +29     
- Misses        329      396     +67     
- Partials       96      105      +9

Impacted Files	Coverage Δ
testkit/util/gnmiutil/helper.go	27.61% <71.42%> (ø)

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