[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@991dd63). Click here to learn what that means.
The diff coverage is n/a.

@@           Coverage Diff           @@
##             main       #2   +/-   ##
=======================================
  Coverage        ?   92.00%           
=======================================
  Files           ?        1           
  Lines           ?       75           
  Branches        ?        3           
=======================================
  Hits            ?       69           
  Misses          ?        6           
  Partials        ?        0           

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 991dd63...b01abdb. Read the comment docs.