JUnit-friendly replacement for MultipleCacheManagerTest

Core tests that need one more more cache managers extend either SingleCacheManagerTest or MultipleCacheManagersTest. Many other modules' tests do the same for convenience.

We would like to eventually move all modules to JUnit, and having an alternative to MCMT that can be used from either JUnit or TestNG and can support an incremental migration is the first step.

The starting plan is for a TestCluster class for building and interacting with clusters including features from MCMT/TestingUtil/TestCacheManagerFactory/BasePartitionHandlingTest.

Assertions like assertLocked(), assertIsInL1() etc. would be in separate classes, ideally with AssertJ-like assertThat static methods.