Reconsider using soft references for `ContextCache` in the TestContext framework

[chschu]

The idea of using soft references to implement a memory-sensitive ContextCache already came up a while ago in #12343. It was implemented and immediately reverted for not properly closing the evicted contexts. With finalize considered bad practice, there was no good way to implement that reliably back then.

However, that changed with JDK 9: The java.lang.ref.Cleaner interface can be used to clean up phantom reachable objects.

It would be a great improvement to have a memory-sensitive ContextCache. No longer having to use @DirtiesContext for memory-heavy tests would be a real charm.