app/test: only build what has been selected in config

app/test: only build what has been selected in config

Avoid building tests if their counterparts are not selected in config.
This has the nice side effect of fixing build errors when disabling parts of
the dpdk.

Signed-off-by: David Marchand <david.marchand@6wind.com>
Acked-by: Bruce Richardson <bruce.richardson@intel.com>
Acked-by: Neil Horman <nhorman@tuxdriver.com>