- Added a new class `ExtendedContainer` that extends League's container.
- `add` modified to reject classes not in the root Woo namespace.
- Has two new methods, `replace` and `reset_resolved`.
- It's used as the underlying container instead of League's one
in `Container`, but the new methods are not exposed.
- At unit test bootstrap time the globally registered container is
replaced with the extended one that `Container` stores
(grabbed from private property using reflection).
- A new `MockableLegacyProxy` is added. It inherits from `LegacyProxy`
and allows to mock functions, static methods and legacy classes.
- The registeed `LegacyProxy` is replaced with the mockable version
during unit test bootstrap.
- A PHPUnit hook is added to reset the mockable proxy to its initial
state (so that nothing is mocked) before each test.
- `WC_Unit_Test_Case` gets helper methods to mock functions, static
methods and classes without having to retrieve the proxy class.