testFailedTxnAsPartOfQuorumLoss is flaky

According to the dashboard testFailedTxnAsPartOfQuorumLoss is flaky. I've looked at some logs and there are multiple causes of flakiness. One of them is in this line after step 5

Assert.assertEquals(1, outstanding.size());

For example this build of 3.5

I was able to reproduce this particular issue in debug mode and the problem is that 'outstading' map can also have 'closeSession' entries that are expected.

I'll submit a patch to relax this check.