MB-8677 Improve test coverage

Add assertion for the exact condition that triggered an error
in MB-8677.

Change-Id: Iae29ff79cfeee413777a7dae223a85c1276c658d
Reviewed-on: http://review.couchbase.org/28110
Reviewed-by: Volker Mische <volker.mische@gmail.com>
Reviewed-by: Filipe David Borba Manana <fdmanana@gmail.com>
Tested-by: Filipe David Borba Manana <fdmanana@gmail.com>