The gptransfer test used a step that looked for a logfile with a date in the
name. If that logfile existed at 11:59PM on the day before, and the test looked
for it at 12:00AM on the next day, it "wouldn't be there"

Refactor the test so that assertions about using the typical
gpAdminLogs directory are as banal as possible.

Also see https://github.com/greenplum-db/gpdb/pull/4072/commits/84bf83d5013f891547dc21576d04a281cfd2faf7.

Author: Nadeem Ghani <nghani@pivotal.io>
Author: Shoaib Lari <slari@pivotal.io>