Makefile: Allow overriding GOFILES.
Makefile: Allow overriding GOFILES.

This allows specifying individual test directories to minimize the
number of unit tests to run, eg:

$ GOFILES="github.com/cilium/cilium/pkg/k8s" make tests

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

eloycoto/cilium,tgraf/cilium,eloycoto/cilium,gumpt/cilium,gumpt/cilium,eloycoto/cilium,scanf/cilium,tgraf/cilium,eloycoto/cilium,tklauser/cilium,cilium-team/cilium,gumpt/cilium,scanf/cilium,cilium/cilium,cilium/cilium,gumpt/cilium,michi-covalent/cilium,scanf/cilium,tgraf/cilium,gumpt/cilium,scanf/cilium,scanf/cilium,michi-covalent/cilium,tgraf/cilium,cilium-team/cilium,michi-covalent/cilium,cilium/cilium,tklauser/cilium,michi-covalent/cilium,tklauser/cilium,tgraf/cilium,eloycoto/cilium,tgraf/cilium,cilium/cilium,tklauser/cilium,eloycoto/cilium,cilium/cilium,scanf/cilium,eloycoto/cilium,scanf/cilium,tklauser/cilium,gumpt/cilium,gumpt/cilium,michi-covalent/cilium