Add more tests and fix upstream throw behaviour

Author: FranzBusch

Sources/AsyncAlgorithms/Merge/AsyncMerge2Sequence.swift

@@ -79,8 +79,8 @@ extension AsyncMerge2Sequence: AsyncSequence {
     }
 }
 
-public extension AsyncMerge2Sequence {
-    struct AsyncIterator: AsyncIteratorProtocol {
+extension AsyncMerge2Sequence {
+    public struct AsyncIterator: AsyncIteratorProtocol {
         /// This class is needed to hook the deinit to observe once all references to the ``AsyncIterator`` are dropped.
         ///
         /// If we get move-only types we should be able to drop this class and use the `deinit` of the ``AsyncIterator`` struct itself.

Sources/AsyncAlgorithms/Merge/MergeStateMachine.swift

@@ -92,12 +92,12 @@ struct MergeStateMachine<
             // An iterator was created and we deinited the sequence.
             // This is an expected pattern and we just continue on normal.
             // Importantly since we are a unicast sequence no more iterators can be created
-            break
+            return
 
         case .finished:
             // We are already finished so there is nothing left to clean up.
             // This is just the references dropping afterwards.
-            break
+            return
 
         case .modifying:
             preconditionFailure("Invalid state")
@@ -160,7 +160,7 @@ struct MergeStateMachine<
         case .upstreamFailure:
             // The iterator was dropped which signals that the consumer is finished.
             // We can transition to finished now. The cleanup already happened when we
-            // transitioned to `upstreamThrew`.
+            // transitioned to `upstreamFailure`.
             state = .finished
 
             return .none