Makefile: Fix PRIV_TEST_PKGS test selection
Makefile: Fix PRIV_TEST_PKGS test selection

Previously this could echo the concatenated set of all TESTPKGS which
meant that the grep couldn't find any privileged files and would pass an
empty string along to the dirname execution which would just print this
warning and result in an empty set of privileged tests:

  dirname: missing operand
  Try 'dirname --help' for more information.

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

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