Add test for lcb_refresh_config

This test doesn't actually check for anything, except that we don't
crash when trying to refresh the config manually at the user's behest.

Change-Id: I67d205df327e94b0788cb3188383334472adb71b
Reviewed-on: http://review.couchbase.org/42547
Tested-by: Mark Nunberg <mnunberg@haskalah.org>
Reviewed-by: Sergey Avseyev <sergey.avseyev@gmail.com>