Add: WPAcceptance test cases

[dinhtungdu]

Description of the Change

This PR:

• Updates WP version of the testing snapshot to latest.
• Add nightly snapshot to test against the development version of WP.
• Adds more test cases to verify plugin functionalities.

Checklist:

• I have read the CONTRIBUTING document.
• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have added tests to cover my change.
• All new and existing tests passed.

Applicable Issues

#205

Changelog Entry

[jeffpaul]

@dinhtungdu let's bump the snapshot to WordPress 5.5 to cover that core upgrade coming this week

[jeffpaul]

• switch to using GitHub Actions
• add daily cron

[helen]

Next steps: @dinhtungdu will be switching to GitHub Actions as the runner and setting up an additional daily cron run (especially useful for nightlies).

[dinhtungdu]

I switched PHPUnit and WPAcceptance tests to GitHub Actions. Daily cron is set up as well. WPAcceptance test is failing because WPsnapshots has a bug with nightly version which is addressed here: 10up/wpsnapshots#78.

[jeffpaul]

@dinhtungdu maybe we revert the nightly part in here so this PR can get merged in and add that back in once it's available in WP Snapshots?

[dinhtungdu]

@jeffpaul Update and GH actions are passing now. This is ready to be merged.