Rxjava: 3.x: Test consumer method renames

[akarnokd]

The feedback for the test consumers called for a few method renames (#6153):

• assertResult - test for values received followed by completion
• assertFailure - test for values received followed by error
• assertEmpty - test for no onNext/onError/onComplete signals
• assertValuesOnly - test for values received but no termination
• awaitDone - wait for termination via blocking or report timeout

Suggestions for better, more apt and hopefully not too long names are welcome.

[vanniktech]

I like the names as they are.

[maryangmin]

I like it too.