Fix compiler warning in mock-unit-test.cc

warning: deprecated conversion from string constant to ‘char*’

Change-Id: I6a0ea3eb6321ae7c9d04965d83f98236b9cdf32b
Reviewed-on: http://review.couchbase.org/29037
Tested-by: Sergey Avseyev <sergey.avseyev@gmail.com>
Reviewed-by: Trond Norbye <trond.norbye@gmail.com>