hack/make: fix test-unit target to propogate TESTFLAGS properly

`TESTFLAGS=-test.v make test-unit` currently does not echo the (very useful) output of `go test -v`. This fixes it.

/cc @tianon 
So, just to make this slightly more concrete, the final line would be:
```bash
if ! ( cd "$dir" && "$precompiled" $TESTFLAGS ); then
```