Improve debugging information on ID uniqueness failure in isolated connection test

Summary: @jungejason reported seeing test failures here. I can't reproduce them
and my read of the code doesn't suggest why they might be happening, but add a
little more debug info in hopes of chasing this down.

Test Plan:

• Ran test in a loop for a long time, couldn't get it to fail.
• Changed assertEquals() condition to force test to fail, verified output

message was informative.

Reviewers: jungejason, btrahan

Reviewed By: jungejason

CC: aran, epriestley, jungejason

Differential Revision: 1212