[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 (main@4b32e77). Click here to learn what that means.
The diff coverage is n/a.

@@           Coverage Diff            @@
##             main       #46   +/-   ##
========================================
  Coverage        ?   100.00%           
========================================
  Files           ?         1           
  Lines           ?        55           
  Branches        ?         6           
========================================
  Hits            ?        55           
  Misses          ?         0           
  Partials        ?         0           

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