For legacy reasons, a MockEnvironment implementation already exists in multiple places within Spring's test suite; however, it is not available to the general public.
This commit promotes MockEnvironment to a first-class citizen in the spring-test module, alongside the existing MockPropertySource.
In addition, the following house cleaning has been performed.
- deleted MockPropertySource from the spring-expression module
- deleted MockEnvironment from the "spring" integration testing module
- updated test copies of MockPropertySource and MockEnvironment
- documented MockEnvironment and MockPropertySource in the testing
chapter of the reference manual
Issue: SPR-9492