Upgrade tests to PHPUnit 12

Summary:
Tests are now organized in a namespace Waystone\Workspaces\Tests,
each class, trait or interface in a dedicated file to follow PSR-4.

Coverage is defined by attributes.

Exceptions are directly handled by PHPUnit assert methods.

Tests can be run from:

• the tests/ folder, as before
• the repository root, with the new /phpunit.xml

A tests bootstrap script allows to load Composer dependencies
from the vendor/ directory at workspaces level or monorepo level.

Test Plan:

• Top-level: phpunit
• Tests-level: (cd workspaces/tests && make test)

Differential Revision: https://devcentral.nasqueron.org/D3760