Fix plot_param_importances test.

[hvy]

Motivation

Unit test for plot_param_importances was fragile as it could fail depending on the evaluated importance (order).

Description of the changes

Fix the test to not consider the order.

[codecov-commenter]

Codecov Report

Merging #1328 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #1328   +/-   ##
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  Coverage   86.69%   86.69%           
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  Files          95       95           
  Lines        7412     7412           
=======================================
  Hits         6426     6426           
  Misses        986      986           

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[toshihikoyanase]

LGTM.

[note] We may be able to fix the order of parameters if we carefully design trials, but I feel it is overengineering for these test cases.

[HideakiImamura]

LGTM.