chore(Spanner): fix mockspanner

Author: harshachinta

google-cloud-spanner/src/test/java/com/google/cloud/spanner/MockSpannerServiceImpl.java

@@ -2014,7 +2014,9 @@ public void commit(CommitRequest request, StreamObserver<CommitResponse> respons
       return;
     }
     sessionLastUsed.put(session.getName(), Instant.now());
-    if (session.getMultiplexed() && !request.hasPrecommitToken()) {
+    if (session.getMultiplexed()
+        && !request.hasPrecommitToken()
+        && !request.hasSingleUseTransaction()) {
       throw Status.INVALID_ARGUMENT
           .withDescription(
               "A Commit request for a read-write transaction on a multiplexed session must specify a precommit token.")