Consider avoiding ending sessions the shell has opened at the end of resmoke test

After a test has completed, we end all of the sessions the shell has opened. If a test creates a large number of sessions, this can take a really long time - i.e. bulk_shard_insert.js inserts 900 docs, and the check_orphans_are_deleted.js hook that runs at the end opens a session per document. We sometimes timeout as seen is BF-16239. This ticket is to look into how long we typically spend ending sessions at the end of a test and to consider skipping doing this to speed up test time.