First, you should seriously question whether writing tests in mocha makes sense. Admittedly, there might be cases when it does make sense (i.e. if you're porting a nodejs app to Go, and already have tests written for node). But even then, you should consider this a stop-gap measure, and write all new tests, and even migrate old tests, to Go as soon as possible.

But that aside, there's no reason you shouldn't be able to just start the process in the background normally. Perhaps with a shell script called from your Travis config (can often be cleaner and easier to follow than putting all commands in the config directly):

#!/bin/sh

go run &

mocha

If you really must run your tests in an external process, there are certain advantages from starting that from a Go test anyway. Namely, you can get test coverage stats, and starting can be more easily synchronized (so you don't need a sleep). To do this, you can follow my advice in this answer. Specifically, for the mocha case:

Write a test file that executes your main() function in a go routine:

func TestMainApp(t *testing.T) {
    go main() // Or whatever you need to do to start the server process
    cmd := exec.Command("mocha", ...)
    cmd.Start()
}

But seriously. You should write your tests in Go.

Mocha runs directly after - nohup go run ./cmd/server/main.go --scheme=http --port=8080 --host=127.0.0.1 & so the server didn't start up yet.

Adding a sleep was enough.

script:
  - npm install mocha -g
  - npm install
  - nohup go run ./cmd/server/main.go --scheme=http --port=8080 --host=127.0.0.1 &
  - sleep 4
  - mocha

But, the point is taken, I will move to Go tests :-P