[#75804242] Consolidate documentation on how to run the tests

Make our documentation consistent so that we only document one way of running the tests.

Do this by replacing the Makefile with a stub bash script and update the Travis CI configuration accordingly.
I wonder if renaming `common_setup_test.go` to `main.go` would fix that `go get` error. I was thinking of doing it anyway, so that the entry point is obvious and it looks a bit more like a normal Go project.