Increase the election timeout period in atlas_snapshot_workflows.js

atlas_snapshot_workflows.js uses a small election timeout period to speed up the test. However, this can also increase the frequency of unexpected step-downs. We should increase the election timeout slightly.

Also, the checkReplicaSets method is always called with a calculation of the live secondaries appearing right before it is called, such as here. This can be moved inside the method, after the getPrimary call to improve synchronization. This isn't necessary to fix the bug, but can be investigated.