Fix unit-test of coco evaluator

[zhiqwang]

No description provided.

[codecov]

Codecov Report

❗ No coverage uploaded for pull request base (master@5a56c16). Click here to learn what that means.
The diff coverage is 100.00%.

@@            Coverage Diff            @@
##             master      #91   +/-   ##
=========================================
  Coverage          ?   83.31%           
=========================================
  Files             ?        8           
  Lines             ?      773           
  Branches          ?        0           
=========================================
  Hits              ?      644           
  Misses            ?      129           
  Partials          ?        0           

Flag	Coverage Δ
unittests	83.31% <100.00%> (?)

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Impacted Files	Coverage Δ
test/test_engine.py	100.00% <100.00%> (ø)

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