test/common: check if pid is alive before kill

Need to check if process still exists before killing it.
Otherwise if process crashed and no longer exists kill will
return with "no such process" error and prevent other steps
in trap from executing.

Change-Id: I4402d06a4fafa09dc13fba4ef9ebeb2b2fc686bb
Signed-off-by: Karol Latecki <karol.latecki@intel.com>
Reviewed-on: https://review.gerrithub.io/425353


Chandler-Test-Pool: SPDK Automated Test System <sys_sgsw@intel.com>
Tested-by: SPDK CI Jenkins <sys_sgci@intel.com>
Reviewed-by: Paweł Niedźwiecki <pawelx.niedzwiecki@intel.com>
Reviewed-by: Shuhei Matsumoto <shuhei.matsumoto.xt@hitachi.com>
Reviewed-by: Jim Harris <james.r.harris@intel.com>
Reviewed-by: Ben Walker <benjamin.walker@intel.com>