test/nvme: Make sure that $rootdir is declared

perf/common.sh uses autotest's common tooling which depends on this
variable to be set. Without it, execution of the run_perf.sh simply
fails.

Change-Id: Ib68d565b1d38c80df3d3f1a09f5fa7bef6eaaffd
Signed-off-by: Michal Berger <michalx.berger@intel.com>
Reviewed-on: https://review.spdk.io/gerrit/c/spdk/spdk/+/1290


Tested-by: SPDK CI Jenkins <sys_sgci@intel.com>
Reviewed-by: Shuhei Matsumoto <shuhei.matsumoto.xt@hitachi.com>
Reviewed-by: Jim Harris <james.r.harris@intel.com>
Reviewed-by: Aleksey Marchuk <alexeymar@mellanox.com>
Reviewed-by: Darek Stojaczyk <dariusz.stojaczyk@intel.com>