test/nvmf: increase the delay before identifying kernel target

With the previous 2 seconds sleep we
still used to randomly fail from time
to time.

Change-Id: I99f8c50ba4af87c47bed5a629ddb79f5eb796d4c
Signed-off-by: Dariusz Stojaczyk <dariuszx.stojaczyk@intel.com>
Reviewed-on: https://review.gerrithub.io/391553


Tested-by: SPDK Automated Test System <sys_sgsw@intel.com>
Reviewed-by: Sebastian Basierski <sebastianx.basierski@intel.com>
Reviewed-by: Daniel Verkamp <daniel.verkamp@intel.com>
Reviewed-by: Ben Walker <benjamin.walker@intel.com>
Reviewed-by: Jim Harris <james.r.harris@intel.com>