[Tests] migrate to github actions; run nyc on all tests

[ljharb]

Per ljharb/object.assign#81

travis-ci's new pricing plan, and its defaults, have caused all my ljharb repos to have zero CI whatsoever until December. @travis-ci Support is MIA, so I unfortunately can't rely on it as a service anymore.

I disabled posttest because that runs zuul, and I'm leaving that in travis for the time being.

[codecov-io]

Codecov Report

❗ No coverage uploaded for pull request base (master@1bda7f2). Click here to learn what that means.
The diff coverage is n/a.

@@            Coverage Diff            @@
##             master      #52   +/-   ##
=========================================
  Coverage          ?   97.54%           
=========================================
  Files             ?        3           
  Lines             ?      448           
  Branches          ?      137           
=========================================
  Hits              ?      437           
  Misses            ?       11           
  Partials          ?        0           

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