test/nvme: fix wait for spdk_tgt error code on exit

Just sending TERM signal to the background process doesn't
provide an information about the condition of terminated
process.

To catch failures in the background process killprocess
helper should be used to wait for its termination and
check error.


Change-Id: I294d4493cf69883aa481eb6f5e55b46ea4940c5c
Signed-off-by: Tomasz Kulasek <tomaszx.kulasek@intel.com>
Reviewed-on: https://review.gerrithub.io/c/spdk/spdk/+/475017


Tested-by: SPDK CI Jenkins <sys_sgci@intel.com>
Reviewed-by: Jim Harris <james.r.harris@intel.com>
Reviewed-by: Ben Walker <benjamin.walker@intel.com>
Reviewed-by: Shuhei Matsumoto <shuhei.matsumoto.xt@hitachi.com>
Reviewed-by: Tomasz Zawadzki <tomasz.zawadzki@intel.com>