Add Pitest as CI with artifact

[wltan]

Pitest is a resource intensive operation when run locally. This allows reports to be generated remotely.

• On every push (e.g. to master main), this job maintains a reference report as a basis for comparison when reviewing pull requests.
• On every pull request, this job generates the latest report for reviewers to download.
  • You can find it under the 'Checks' tab here:

It is run as a separate job, so as not to slow down the other verification jobs.

Test run for pushing: https://github.com/wltan/tp/runs/1368073912 Test run for pull request: (this PR, under 'Checks')

[codecov-io]

Codecov Report

Merging #296 (73ccc4a) into master (8e535b6) will not change coverage. The diff coverage is n/a.

@@            Coverage Diff            @@
##             master     #296   +/-   ##
=========================================
  Coverage     88.26%   88.26%           
  Complexity      951      951           
=========================================
  Files           120      120           
  Lines          2334     2334           
  Branches        319      319           
=========================================
  Hits           2060     2060           
  Misses          168      168           
  Partials        106      106           

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