fix(spanner): unflake unit tests

Author: sakthivelmanii

google-cloud-spanner/src/main/java/com/google/cloud/spanner/ResumableStreamIterator.java

@@ -68,7 +68,7 @@ abstract class ResumableStreamIterator extends AbstractIterator<PartialResultSet
   private final int maxBufferSize;
   private final ISpan span;
   private final TraceWrapper tracer;
-  private CloseableIterator<PartialResultSet> stream;
+  private volatile CloseableIterator<PartialResultSet> stream;
   private ByteString resumeToken;
   private boolean finished;
   /**