Tests and related fixes to the TCK, "tests of the tests"

self pull requesting for initial comments and cleaning up.
Will PR to reactive-streams in an hour or two (once cleaned up a bit).

// cc @viktorklang 
This is `spec212_mustNotCallOnSubscribeMoreThanOnceBasedOnObjectEquality`, and the spec states that: [2.12](https://github.com/reactive-streams/reactive-streams#2.12) Subscriber.onSubscribe MUST NOT be called more than once (based on object equality)

so this test is over doing it I guess. Should we remove? // cc @viktorklang