Bug 1070442 - Support building gaia apps in parallel

Ah, this is because I have some unit test leftovers from old builds:
```
gaia$ ls apps/browser/ -R
apps/browser/:
test

apps/browser/test:
integration  unit

apps/browser/test/integration:

apps/browser/test/unit:
_proxy.html  _sandbox.html
```
But I think it is still worth fixing that issue as I may not be the only one with such setup.