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 was a legitimate bug - the test is about checking `onError` (`spec203_mustNotCallMethodsOnSubscriptionOrPublisherInOnError`), yet it called `onComplete`!
OnComplete is covered in another test