vm: add tests to stack_item.go

[AnnaShaleva]

closes #199

[codecov]

Codecov Report

Merging #812 into master will increase coverage by 0.56%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #812      +/-   ##
==========================================
+ Coverage   67.08%   67.65%   +0.56%     
==========================================
  Files         141      141              
  Lines       13204    13204              
==========================================
+ Hits         8858     8933      +75     
+ Misses       3930     3859      -71     
+ Partials      416      412       -4     

Impacted Files	Coverage Δ
pkg/vm/stack_item.go	96.16% <0.00%> (+26.13%)	⬆️

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