PerApp test helpers a=test-only r=etienne_s

Allows each much more flexibility when it comes to reusing code across tests.

a quick is example is like this:

```js
// in setup.js
function mockEverything() {
  suiteSetup(function() {
     // do stuff
  });
}

suiteSetup(function() {
 // before all tests etc...
});

suiteTeardown(function() {
 // after all tests...
});


// in tests

suite('my test', function() {
  // this was impossible before
  mockEverything();
});

```