-Wno-missing-field-initializers for tests on cmake

Change-Id: Ic86afe32e717ac557defbfe0a2e985296cd34a67
Reviewed-on: http://review.couchbase.org/27737
Reviewed-by: Sergey Avseyev <sergey.avseyev@gmail.com>
Tested-by: Sergey Avseyev <sergey.avseyev@gmail.com>