improve unit tests

[ubogdan]

Describe the PR
improve code coverage
enable parallel tests

Relation issue
NONE

Additional context
NONE

[codecov]

Codecov Report

Merging #981 (50d4f1e) into master (497e6e2) will increase coverage by 1.90%.
The diff coverage is 98.68%.

@@            Coverage Diff             @@
##           master     #981      +/-   ##
==========================================
+ Coverage   90.59%   92.50%   +1.90%     
==========================================
  Files           8        8              
  Lines        1914     1921       +7     
==========================================
+ Hits         1734     1777      +43     
+ Misses        104       81      -23     
+ Partials       76       63      -13     

Impacted Files	Coverage Δ
parser.go	92.43% <98.61%> (+3.06%)	⬆️
operation.go	94.24% <100.00%> (+1.35%)	⬆️

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