make it easier to mock plugin discovery in tests

Problem/Motivation

Mocking plugin discovery is really painful -- see ReflectionFactoryTest and MigrateFieldPluginManagerTest for instance.

This is something that needs to be tested fairly often though -- things like:

- testing that the plugin manager throws exceptions for certain badly-formed plugin definitions
- testing for deprecations of things in plugin definitions

It would be nice if this were easier -- either by making changes in the plugin system, or providing common helper code for tests to use.

Steps to reproduce

Proposed resolution

Remaining tasks

User interface changes

Introduced terminology

API changes

Data model changes

Release notes snippet