Update `SpringBootContextLoader` so that when possible the
`SpringApplication` remains in control of creating the `Environment`
instance.
Prior to this commit, we would always create the `Environment` in the
`SpringBootContextLoader` and then call `setEnvironment` on the
`SpringApplication`. This meant that the `ApplicationEnvironment`
classes were not used and that `isCustomEnvironment` was set to `true`
so no conversion was applied.
With the updated code, an `ApplicationListener` is used to mutate the
`Environment` instance and add the required test property sources.
Fixes gh-29169
3f7bf7d34f