[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

Merging #8 (799c918) into master (940f3f9) will not change coverage.
The diff coverage is n/a.

@@            Coverage Diff            @@
##            master        #8   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files            1         1           
  Lines           15        15           
  Branches         4         4           
=========================================
  Hits            15        15           

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