Make it possible to run tests in PHP 8

[psrpinto]

Problem

It was not possible to run tests under PHP 8 because PHPUnit's version that we were using (^7) does not support PHP 8. The earliest PHPUnit version that supports PHP 8 is 9.3.

Solution

Upgrade PHPUnit to ^9.6 (9.6 is the latest minor version of PHPUnit 9).

Changes to GitHub actions

GitHub actions have been modified to run tests for multisite on and multisite off, on latest and nightly, and on both 7.4 and 8.3. Coverage runs only on 8.3.

Helper scripts for running tests in wp-env

Two new npm scripts have been added to facilitate running tests in the wp-env's tests-cli container:

• npm run env:tests: Runs composer install and then runs tests
• npm run env:tests-cmd: Facilitates running arbitrary commands in the tests container, e.g. run a single test:
  • npm run env:tests-cmd vendor/bin/phpunit -- --filter test_multiple_imports_singular

To-do

• Upgrade PHPUnit to ^8. PHPUnit 8's release notes.
• Fix issues that caused tests to fail under PHPUnit ^8
• Fix deprecations introduced by PHPUnit ^8
• Upgrade PHPUnit to ^9. PHPUnit 9's release notes.
• Fix deprecations introduced by PHPUnit ^9
• Make sure tests run in wp-env under PHP 8
• Make GitHub workflow also test under PHP 8

Testing Instructions

1. npm run env:start
2. npm run env:tests

[psrpinto]

This is ready for review. All checks in the PR are passing, it's not marked as "checked" because repository settings need to be changed so that required checks are not using PHP 7.4 (please see To-do in PR description).

[akirk]

Thank you!