example.js

grunt.initConfig({
    nodemocha: {
        options: {
            bin: './node_modules/.bin/', // to use locally-installed mocha
            args: [
                'test/lib/*.coffee',
                'test/mailer/*.coffee'
            ]
        }
    }
});

nodemocha.js

module.exports = function(grunt) {
    'use strict';

    var path = require('path'),
        exec = require('child_process').exec;

    grunt.registerTask('nodemocha', 'Run node tests with mocha', function() {
        var bin, cmd, proc,
            options = this.options(),
            done = this.async();
        bin = path.join((options.bin || ''), 'mocha');
        cmd = [bin].concat(options.args || []).join(' ');
        proc = exec(cmd, function(err) {
            done(err);
        });
        proc.stdout.pipe(process.stdout);
    });
};