[Tests] migrate tests to Github Actions

[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.

[codecov]

Codecov Report

❗ No coverage uploaded for pull request base (master@7aef399). Click here to learn what that means.
The diff coverage is n/a.

@@            Coverage Diff            @@
##             master       #7   +/-   ##
=========================================
  Coverage          ?   88.88%           
=========================================
  Files             ?        5           
  Lines             ?       72           
  Branches          ?       13           
=========================================
  Hits              ?       64           
  Misses            ?        8           
  Partials          ?        0           

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