rewind stream in test before reading

Previously a helper method accomplished this. Now, the library uses
standard stream-copy APIs, and must remember to also rewind when needed.
Fixes live-only test where this rewind was forgotten in Azure#49416.

Author: jaschrep-msft

sdk/storage/Azure.Storage.Common/tests/PooledMemoryStreamTests.cs

@@ -71,7 +71,7 @@ public void StreamCanHoldLongData()
             PredictableStream originalStream = new PredictableStream();
             PooledMemoryStream arrayPoolStream = new(_pool, bufferPartitionSize);
             originalStream.CopyToExactInternal(arrayPoolStream, dataSize, async: false, cancellationToken: default).EnsureCompleted();
-            originalStream.Position = 0;
+            arrayPoolStream.Position = 0;
 
             // assert it holds the correct amount of data. other tests assert data validity and it's so expensive to do that here.
             // test without blowing up memory