Add extra Maybe iterable Amb tests for overlapped emissions

[rzsombor]

• Asserting that downstream will get the results of the first source that began emission, even if multiple sources have overlapped emissions
• Testing Success and Error paths

[rzsombor]

As far as I can see the CI error is coming from a seemingly unrelated test:

io.reactivex.internal.operators.maybe.MaybeUnsubscribeOnTest > normal FAILED
java.lang.AssertionError
    at org.junit.Assert.fail(Assert.java:86)
    at org.junit.Assert.assertTrue(Assert.java:41)
    at org.junit.Assert.assertFalse(Assert.java:64)
    at org.junit.Assert.assertFalse(Assert.java:74)
    at io.reactivex.internal.operators.maybe.MaybeUnsubscribeOnTest.normal(MaybeUnsubscribeOnTest.java:50)

I'm a bit confused here - did I really manage to break this test or is this one known to be flaky?

[akarnokd]

It was a flaky test: the unsubscription may not reach the processor in time the latch unblocks the main thread and thus there appear to be subscribers there.

👍 Thanks for the unit tests.