Bootstrap WP CLI in PHPUnit environment

[johnwatkins0]

Summary

Fixes #5523 by making WP CLI utility functions available in the PHPUnit environment. Currently, PR #5306 is failing with a fatal error due to a missing WP CLI utility function, and this will fix that.

To fix it, I created a bootstrap.php file inside tests/php and from there imported the wp-dev-lib bootstrap file along with WP CLI's own PHPUnit bootstrap file. As @schlessera suggested on Slack, I also added wp-cli/wp-cli as a dev dependency with Composer to make sure it's actually there.

Checklist

• My pull request is addressing an open issue (please create one otherwise).
• My code is tested and passes existing tests.
• My code follows the Engineering Guidelines (updates are often made to the guidelines, check it out periodically).

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[pierlon]

This looks fine to me, .