Set WordPress minimum version to 5.9 & update PHPUnit

[alecgeatches]

This PR makes a handful of changes to PHP unit tests to run with phpunit ^9.0:

• Simplified test matrix to PHP 7.4/WP 5.9 (lowest support), and PHP latest/WP latest.

• Use wp-env for setting up WordPress/testing environment rather than bin/install-wp-tests.sh and friends. I based this on previous work in another repository. This simplifies running tests in GitHub Actions and locally to work the same:

  wp-env start
  composer run integration

• Removed wp-env-specific configuration from package.json. The docker compose overrides did not appear necessary nor did the rest of the configuration.

• Updated the tests bootstrap.php file to the latest from wp scaffold plugin-tests, and also added a small change to enable email-related tests to work.

• Added a bunch of protected and : void return types to unit test functions (e.g. setUp(), tearDown()) to match PHPUnit 9's parent classes.

• Changed some ->assertContains() calls to ->assertStringContainsString() when used in a string context. assertContains() for strings was deprecated in PHPUnit 8.

• Removed behat-test.yml. Apparently this test was added by a bot last year. The referenced workflow checks for a behat.yml file before running, and I can't find any reference to behat configuration ever in this repository. These were previously causing alerts for "cancelled" checks on each test run: