guard against stale container problems in tests

Problem/Motivation

When you do something in a test which causes a container rebuild, the test class still have a stale container in $this->container.

This is an easy trap for test developers to fall into.

But we could easily mitigate it by warning of an attempt to access a stale container.

Steps to reproduce

Proposed resolution

Add a subclass or wrapper class of Container, to use in tests.

This adds:

protected bool $isStale;

public function setStale();

and wraps the call to get() with a check for whether it's stale, and throws an exception if it is.

Remaining tasks

User interface changes

Introduced terminology

API changes

Data model changes

Release notes snippet