Turn on ObjectReuse in MultipleProgramsTestBase:
TestEnvironment clusterEnv = new TestEnvironment(cluster, 4, true);
Then the tests "testEventTimeSessionGroupWindow", "testEventTimeSessionGroupWindow", and "testEventTimeTumblingGroupWindowOverTime" will fail.
The reason is that we have buffered iterated records for group-merge. I think we should change the Agg merge to pair-merge, and later add group-merge when needed (in the future we should add rules to select either pair-merge or group-merge, but for now all built-in aggregates should work fine with pair-merge).