Update `SpringConfigurationPropertySource` so that the
`SystemEnvironmentPropertyMapper` is only used for the "actual" system
environment property source. This allows SystemEnvironmentProperySource
class to be used for other purposes (for example, Spring Cloud uses it
to as an override source providing decryption).

Only property sources named `systemEnvironment` or ending with
`-systemEnvironment` now have the `SystemEnvironmentPropertyMapper`
applied. The `TestPropertyValues` has been retrofitted to name the
source it adds appropriately.

Fixes gh-10840