Add opset option on pytest

[disktnk]

support "opset-versions" option

$pytest --opset-versions=latest tests/functions_tests/test_activations.py

• "latest": test only latest version of onnx (not onnx-chainer) library, if use onnx==1.5, use 10
• "min": test only minimum version supported by onnx-chainer, currently 7
• list phrase: test on specialized versions, for example, set "7,10" use upset version 7 and 10

[codecov-io]

Codecov Report

Merging #229 into master will increase coverage by 0.25%. The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #229      +/-   ##
==========================================
+ Coverage   90.62%   90.87%   +0.25%     
==========================================
  Files          24       24              
  Lines        1578     1578              
==========================================
+ Hits         1430     1434       +4     
+ Misses        148      144       -4

Impacted Files	Coverage Δ
onnx_chainer/functions/math.py	89% <0%> (+2%)	:arrow_up:

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