Stop calling 'make' from 'go test'

Because workhorse relies on helper executables, we used to call make from within some of our tests to make sure the executables exist during testing. This adds around 20 seconds of overhead to go test gitlab.com/gitlab-org/gitlab-workhorse on my machine.

This change makes it so that we check during test setup if the executables exist. If they don't we error out asking the user to run the right make command.