introduce wdio for e2e tests

[diosmosis]

Description:

Refs MWP-166

Changes:

• Introduce wdio and add one single e2e test for tracking.
• Add package.json that was missing.
• Add new WP CLI commands: install, sync (for users or sites), getting/setting global settings.
• Fix issue in Installer.php: activate_$plugin hook can be invoked with the path to the plugin entrypoint (ie, matomo/matomo.php). This happens, for instance, when activating the plugin via wp-cli. Currently this change doesn't have an effect though, since the Installer class is only initiated in very specific circumstances.
• In local dev environment, use wordpress.org API to get the latest version.
• In local dev environment, allow using folder that doesn't match wordpress version name, eg, test.
• In local dev environment, allow dropping the existing database if any, to start with a clean setup (for tests).
• In local dev environment, make sure debug.log can be modified by a developer without them having to run a chmod command.
• In local dev environment, use wp-cli to create application password to use for WP rest API.

Currently the tests pass locally. Another PR will be created to run the test in github actions and add some README contents.

Review

• Functional review done
• Potential edge cases thought about (behavior of the code with strange input, with strange internal state or possible interactions with other Matomo subsystems)
• Usability review done (is anything maybe unclear or think about anything that would cause people to reach out to support)
• Security review done
• Wording review done
• Code review done
• Tests were added if useful/possible
• Reviewed for breaking changes
• Developer changelog updated if needed
• Documentation added if needed
• Existing documentation updated if needed