Make `spring-boot-test-autoconfigure` optionally depend on
`spring-boot-autoconfigure` so that it can be an `api` dependency
for `*-test` modules. Also relocate it to core.
See gh-47322
This commit modularizes spring-boot-test-autoconfigure. It now
contains only the code that's central to test auto-configuration.
Feature-specific functionality has moved out into -test modules,
some existing and some newly created. For example, `@DataJpaTest` can
now be found in spring-boot-data-jpa-test.
Closes gh-47322
Stéphane Nicoll
Andy Wilkinson
Moritz Halbritter
Phillip Webb