Update wp-env to version 9.0

[imath]

This is a task I skipped on purpose in #9047 as wp-env's way of testing PHP has evolved. After digging more into it, I wasn't able to find the right use for wp-env run tests-cli phpunit as explained ​here.

But I was able to use the wp-env run cli --env-cwd=wp-content/plugins/buddypress composer command to run PHPUnit tests.

I'll submit a PR to run the GH actions to see if my way of testing PHP with wp-env is also fine for these.

• Updates package.json @wordpress/env version and PHPUnit related scripts
• Adds a script to ease multisite PHPunit testing in composer.json
• Fixes some failing tests in wp-env due to the way the site domain is defined.

PS: some constant/config files may not be needed anymore (eg: BP_USE_WP_ENV_TESTS or env.xml).

Trac ticket: ​https://buddypress.trac.wordpress.org/ticket/9053

Unit Testing when WordPress version is < 6.1 still needs some work and seems to be the only case where we need the wp-phpunit/wp-phunit composer package and constant/config files like BP_USE_WP_ENV_TESTS or env.xml.

[imath]

In 13720:

Update wp-env to version 9.0.0 & adapt the PHPUnit tests suite to it

• Update PHPUnit related npm scripts for wp-env 9.0.0.
• Create test_multi: a new composer script to run Multisite Unit tests.
• Remove our composer dependency to wp-phpunit/wp-phpunit.
• Update the PHPUnit tests suite so that it stops using wp-phpunit/wp-phpunit when it's within the wp-env context.
• Remove no more used PHPUnit specific configuration files.

Fixes #9053
Closes ​https://github.com/buddypress/buddypress/pull/211