Convert the following secondary snapshot reads tests to use multi-statement transactions:

• readConcern_atClusterTime_noop_write.js
  This gives essential coverage for issuing a noop write for atClusterTime on secondaries.
• readConcern_atClusterTime_snapshot_selection.js
  This gives essential coverage for snapshot selection using atClusterTime on secondaries.
• snapshot_reads.js
  This is general coverage for snapshot reads on primaries and secondaries.