Make some white-box tests less flaky

If we want the *internal* state of the oplog driver to be consistent, we
need to make sure that we start our observes at a consistent point in
the oplog.  (ie, initial inserts need to have been fully processed, so
that we don't process them during the original QUERYING phase, which can
give us a different unpublished buffer.)

Note that only the white-box tests (looked at _unpublishedBuffer, eg)
appeared to be flaky: the actual docs published seemed to be correct in
all cases.