Introduce new 'spring-build-junit' subproject containing shared
JUnit utilities and classes to be used by other test cases. This
project is for internal use within the framework, and therefore
creates no artifacts to be published to any repository.
The initial code includes support for JUnit Assumptions that can
be used to determine when a test should run. Tests can be skipped
based on the running JDK version, logging level or based on specific
'groups' that have activated via a Gradle property.
It is intended that sources within the spring-build-junit project be
folded into spring-core/src/test/java, pending some Gradle work that
will facilitate sharing test output across subprojects; therefore this
commit should be seen as a temporary solution.
Issue: SPR-9984
b083bbdec7