[Bug] Skip oracle slashing for the not passed ballot

[yun-yeo]

Summary of changes

• skip oracle slashing for the not passed ballot

close #278

Report of required housekeeping

• Github issue OR spec proposal link
• Wrote tests
• Updated relevant documentation (docs/)
• Added a relevant changelog entry: clog add [section] [stanza] [message]

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(FOR ADMIN) Before merging

• Added appropriate labels to PR
• Squashed all commits, uses message "Merge pull request #XYZ: [title]" (coding standards)
• Confirm added tests are consistent with the intended behavior of changes
• Ensure all tests pass

[codecov]

Codecov Report

Merging #276 into develop will increase coverage by 0.02%.
The diff coverage is 100%.

@@             Coverage Diff             @@
##           develop     #276      +/-   ##
===========================================
+ Coverage    72.01%   72.04%   +0.02%     
===========================================
  Files           72       72              
  Lines         3052     3055       +3     
===========================================
+ Hits          2198     2201       +3     
  Misses         747      747              
  Partials       107      107