a-test.js

var assert = require('assert');
var a = require('./a');

suite("A", function() {
  test("a", function() {
    a.sub(function() {
      assert.ok(true);
    });
    a.trigger();
  });
});

a.js

var targets = [];
module.exports = {
  sub: function(fn) {
    targets.push(fn);
  },

  trigger: function(v) {
    targets.forEach(function(fn) {
      fn(v);
    });
  }
};

b-test.js

var assert = require('assert');
var b = require('./b');

suite("B", function() {
  test("b", function() {
    assert.ok(true);
  });
});

b.js

var assert = require('assert');
var a = require('./a');

a.sub(bad);

function bad() {
  assert(false);
}

run-tests

#!/bin/bash

echo Tests run separately...

mocha --ui tdd a-test
mocha --ui tdd b-test


echo Tests run together...
mocha --ui tdd .