Change assertions in update_shard_key_conflicting_writes.js to be assert.soon

After SERVER-37364 a transaction coordinator will return the commit decision as soon as the decision is persisted and majority committed. There are a number of cases in this test where we run a transaction in a parallel shell, and then make an assertion about the writes done in that transaction outside of the shell. Because this commit made it such that we may not see changes from a transaction without using a session with causal consistency for a bit, we should change any assertions to be an assert.soon. In particular this assertion should be changed (and we should double check for others!).