Allow passing TESTFLAGS env to hack/make.sh from make
Allow passing TESTFLAGS env to hack/make.sh from make

Minor change to allow passing the env TESTFLAGS to make. That's pretty
convenient to filter what tests to run.
E.g. run integration tests containing the substring `Copy`:
make test-integration TESTFLAGS="-check.f Copy"

Signed-off-by: Marco Vedovati <7ad294ca8ed51ab6f7b6cc42294ec572c85df1f5@suse.com>

projectatomic/skopeo,mtrmac/skopeo,mtrmac/skopeo,mtrmac/skopeo,projectatomic/skopeo