test/nvmf: Enable the multicore test.

Purpose: This test will be helpful to find more
issues when running nvmf_tgt with multiple core
configuration.

Change-Id: I06b6bec7a258084a30677356cdf89eb7f61c4ed8
Signed-off-by: Ziye Yang <optimistyzy@gmail.com>
Reviewed-on: https://review.gerrithub.io/401543


Tested-by: SPDK Automated Test System <sys_sgsw@intel.com>
Reviewed-by: Jim Harris <james.r.harris@intel.com>
Reviewed-by: Changpeng Liu <changpeng.liu@intel.com>
Reviewed-by: Shuhei Matsumoto <shuhei.matsumoto.xt@hitachi.com>