make ppo discrete test script more general

[Trinkle23897]

also fix #249 (comment)

[codecov-commenter]

Codecov Report

Merging #418 (4838dc3) into master (bba30f8) will increase coverage by 0.05%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #418      +/-   ##
==========================================
+ Coverage   94.63%   94.69%   +0.05%     
==========================================
  Files          57       57              
  Lines        3749     3749              
==========================================
+ Hits         3548     3550       +2     
+ Misses        201      199       -2     

Flag	Coverage Δ
unittests	94.69% <ø> (+0.05%)	⬆️

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Impacted Files	Coverage Δ
tianshou/policy/modelfree/ppo.py	93.93% <0.00%> (+3.03%)	⬆️

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