ci: Enhance test workflow

[peaceiris]

• remove unused comment
• Add ubuntu-16.04
• bump actions/upload-artifact from v1 to v2
• Fix coverage artifact name

[codecov]

Codecov Report

Merging #198 into master will not change coverage.
The diff coverage is n/a.

@@            Coverage Diff            @@
##            master      #198   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files            5         5           
  Lines          105       105           
  Branches        14        14           
=========================================
  Hits           105       105           

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