policy: Document l4filter test cases
policy: Document l4filter test cases

Add a bit of documentation at the top of this file to give an overview
of the similarities/differences between the different test cases.

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

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