testhelper: run the relevant part of "make prepare-tests" for you

After "git clean -dxff" running "go test -v" will fail saying you need to run "make prepare-tests". Let's just run the relevant "git clone" for the user instead.

Yes, this would be avoided by just running the tests via "TEST_PACKAGES=... make test" or whatever, but let's be more helpful in these one-off "go test -v" cases anyway. It helps if standard Go tooling works as expected out of the box.