test(spanner): Enabled multiplexed session for partitioned operations in systest. (#3545)

Author: pratickchokhani

google-cloud-spanner-executor/src/main/java/com/google/cloud/executor/spanner/CloudClientExecutor.java

@@ -832,6 +832,8 @@ private synchronized Spanner getClient(long timeoutSeconds, boolean useMultiplex
     SessionPoolOptionsHelper.setUseMultiplexedSession(poolOptionsBuilder, useMultiplexedSession);
     SessionPoolOptionsHelper.setUseMultiplexedSessionForRW(
         poolOptionsBuilder, useMultiplexedSession);
+    SessionPoolOptionsHelper.setUseMultiplexedSessionForPartitionedOperations(
+        poolOptionsBuilder, useMultiplexedSession);
     LOGGER.log(
         Level.INFO,
         String.format(