MB-8689 Increase test 27-database-purges.t coverage

Change-Id: I2596e8eed4956af2815d7f4d10f7a1cdd1d7a454
Reviewed-on: http://review.couchbase.org/27891
Reviewed-by: Volker Mische <volker.mische@gmail.com>
Tested-by: Filipe David Borba Manana <fdmanana@gmail.com>