Makefile: Fix coverpkg when specifying TESTPKGS
Makefile: Fix coverpkg when specifying TESTPKGS

The coverpkg parameter was not passing the correct value to go test when
the user specified TESTPKGS, which meant that if developers wanted to
figure out the coverage of a particular package, they'd either have to
run the full set of unit tests or figure out the correct COVERPKG
argument and pass that into "make unit-tests".

Fix it so that COVERPKG inherits the correct package names from TESTPKGS.

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

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