Add extra Maybe iterable Amb tests for overlapped emissions (#4567)

- 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

Author: Zsombor Erdődy-Nagy

src/test/java/io/reactivex/maybe/MaybeTest.java

@@ -1700,6 +1700,30 @@ public void ambIterable2SignalsSuccess() {
         ts.assertResult(2);
     }
 
+    @SuppressWarnings("unchecked")
+    @Test
+    public void ambIterable2SignalsSuccessWithOverlap() {
+        PublishProcessor<Integer> pp1 = PublishProcessor.create();
+        PublishProcessor<Integer> pp2 = PublishProcessor.create();
+
+        TestSubscriber<Integer> ts = Maybe.amb(Arrays.asList(pp1.toMaybe(), pp2.toMaybe()))
+                .test();
+
+        ts.assertEmpty();
+
+        assertTrue(pp1.hasSubscribers());
+        assertTrue(pp2.hasSubscribers());
+
+        pp2.onNext(2);
+        pp1.onNext(1);
+        pp2.onComplete();
+
+        assertFalse(pp1.hasSubscribers());
+        assertFalse(pp2.hasSubscribers());
+
+        ts.assertResult(2);
+    }
+
     @SuppressWarnings("unchecked")
     @Test
     public void ambIterable1SignalsError() {
@@ -1744,6 +1768,29 @@ public void ambIterable2SignalsError() {
         ts.assertFailure(TestException.class);
     }
 
+    @SuppressWarnings("unchecked")
+    @Test
+    public void ambIterable2SignalsErrorWithOverlap() {
+        PublishProcessor<Integer> pp1 = PublishProcessor.create();
+        PublishProcessor<Integer> pp2 = PublishProcessor.create();
+
+        TestSubscriber<Integer> ts = Maybe.amb(Arrays.asList(pp1.toMaybe(), pp2.toMaybe()))
+                .test();
+
+        ts.assertEmpty();
+
+        assertTrue(pp1.hasSubscribers());
+        assertTrue(pp2.hasSubscribers());
+
+        pp2.onError(new TestException("2"));
+        pp1.onError(new TestException("1"));
+
+        assertFalse(pp1.hasSubscribers());
+        assertFalse(pp2.hasSubscribers());
+
+        ts.assertFailureAndMessage(TestException.class, "2");
+    }
+
     @SuppressWarnings("unchecked")
     @Test
     public void ambIterable1SignalsComplete() {