testhelper: Unify building of Go binaries

Some of our tests rely on some Gitaly commands being built. In order to still allow for go test to be used without having to manually build them, we thus have a set of functions in our testhelper which does build them for us, but they all use the same boiler plate without any kind of unification.

Implement a new buildCommand function and refactor existing code to use it to avoid this duplicated code.