Missing instrunctions on how to run the tests

[strk]

Ideally also provide a simple Makefile with a "check" rule, following GNU standards

[xavi-]

You can run the test be cd-ing to the test directory and executing the following commands;

node callbackTest.js
node errorTest.js
node fnTest.js
node groupTest.js
node parallelTest.js

Creating a Makefile is an interesting proposal, but I feel it might lead to confusion, since there's nothing to build in this project or install (npm takes care of that).