dpdkbuild: disable building DPDK tests

DPDK tests were usualy disabled in the DPDK submodule,
as part of skipping dpdk/app directory.

Since DPDK 18.02 -Dtests=false option was added to
skip building tests in dpdk/app/test.
This reduces the overall time taken to build DPDK from
submodule without introducing new changes to DPDK.

Signed-off-by: Tomasz Zawadzki <tomasz.zawadzki@intel.com>
Change-Id: I425976fa38e09c140e517cccd8aeedd64c67b06c
Reviewed-on: https://review.spdk.io/gerrit/c/spdk/spdk/+/10504


Tested-by: SPDK CI Jenkins <sys_sgci@intel.com>
Reviewed-by: Ben Walker <benjamin.walker@intel.com>
Reviewed-by: Aleksey Marchuk <alexeymar@mellanox.com>
Reviewed-by: Paul Luse <paul.e.luse@intel.com>
Reviewed-by: Jim Harris <james.r.harris@intel.com>