test/common: point to $rootdir instead of pwd in get_nvme_bdfs

Tests should always have a reference to $rootdir absolute path.
Before this change, result of this function depended on current
working directory.

Test like test/nvme/spdk_nvme_cli.sh where directory was changed,
would fail.

With this change the path to get_nvme.sh script should always be
correct.

Signed-off-by: Tomasz Zawadzki <tomasz.zawadzki@intel.com>
Change-Id: Id555188f032e179eedd406463446a85e13234b6d
Reviewed-on: https://review.spdk.io/gerrit/c/spdk/spdk/+/2936


Community-CI: Mellanox Build Bot
Tested-by: SPDK CI Jenkins <sys_sgci@intel.com>
Reviewed-by: Changpeng Liu <changpeng.liu@intel.com>
Reviewed-by: Jim Harris <james.r.harris@intel.com>
Reviewed-by: Ben Walker <benjamin.walker@intel.com>
Reviewed-by: Maciej Wawryk <maciejx.wawryk@intel.com>
Reviewed-by: Karol Latecki <karol.latecki@intel.com>