Upload coverage report for JS unit tests to codecov

[mikkamp]

Changes proposed in this Pull Request:

When running the unit tests for JS, coverage reports are already being generated in various formats. This PR uploads the clover format to codecov.io so they can be included in the coverage report comment.

The coverage comment is currently only showing the status for PHP coverage, since there is no base to compare with yet for the JS coverage reports. Over time we'll have to monitor how accurate this is, since JS / PHP coverage reports are run conditionally depending on which files are changed. So not every PR / head commit has both to compare the differences.

Closes #864

Detailed test instructions:

1. Confirm that the JS code coverage results are available: https://app.codecov.io/github/woocommerce/google-listings-and-ads/pull/2238/tree/js/src
2. Confirm that the PHP code coverage results are available: https://app.codecov.io/github/woocommerce/google-listings-and-ads/pull/2238/tree/src

Changelog entry

• Dev - Upload coverage report for JS unit tests to codecov.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (876e3fe) 60.1% compared to head (03853e1) 60.1%.
Report is 3 commits behind head on develop.

Additional details and impacted files

@@            Coverage Diff            @@
##             develop   #2238   +/-   ##
=========================================
  Coverage       60.1%   60.1%           
  Complexity      4141    4141           
=========================================
  Files            454     454           
  Lines          17586   17586           
=========================================
  Hits           10572   10572           
  Misses          7014    7014           

Flag	Coverage Δ
php-unit-tests	60.1% <ø> (ø)

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[puntope]

Thanks for this @mikkamp LGTM