Makefile: Make PRIV_TEST_PKGS configurable
Makefile: Make PRIV_TEST_PKGS configurable

This allows you to specify an env variable to run specific tests:

$ PRIV_TEST_PKGS=pkg/datapath/loader make tests-privileged

Signed-off-by: Joe Stringer <16a9a54ddf4259952e3c118c763138e83693d7fd@cilium.io>

cilium-team/cilium,cilium/cilium,cilium/cilium,michi-covalent/cilium,michi-covalent/cilium,tgraf/cilium,cilium-team/cilium,tgraf/cilium,tgraf/cilium,cilium/cilium,cilium/cilium,tgraf/cilium,tklauser/cilium,tklauser/cilium,tklauser/cilium,tklauser/cilium,tklauser/cilium,michi-covalent/cilium,michi-covalent/cilium,tgraf/cilium,tgraf/cilium,cilium/cilium,michi-covalent/cilium