fix: use separate value generator in test generation

[yihanzhen]

Sometimes a library has two requests methods with a field of the same name but different types. The TestValueGenerator keys on the short name of fields, and reuses a generated value if it has generated a value for this key (short field name). Therefore in such cases it may try to assign a string literal to a integer type.

Change to create a different TestValueGenerator for every single test case generation to avoid such cases.

[codecov]

Codecov Report

Merging #3294 into master will not change coverage.
The diff coverage is 100.00%.

@@            Coverage Diff            @@
##             master    #3294   +/-   ##
=========================================
  Coverage     87.14%   87.14%           
  Complexity     6105     6105           
=========================================
  Files           495      495           
  Lines         24146    24146           
  Branches       2628     2628           
=========================================
  Hits          21041    21041           
  Misses         2242     2242           
  Partials        863      863           

Impacted Files	Coverage Δ	Complexity Δ
...e/api/codegen/transformer/TestCaseTransformer.java	98.59% <100.00%> (ø)	54.00 <0.00> (ø)

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[vam-google]

LGTM, thanks Hanzhen for fixing the stuff you are not longer responsible for =)