Add AspectMock for mocking built-in functions

It's possible to do this without the library, but the library makes it
simpler, so we don't have to have a bunch of extra files in the test code
base for each of the mocked methods in the namespace, and then manually
require them. This is much cleaner and I think makes it more obvious what
the mock is doing.

composer.json

@@ -25,7 +25,8 @@
         "source": "https://github.com/ramsey/uuid"
     },
     "require": {
-        "php": ">=5.4"
+        "php": ">=5.4",
+        "codeception/aspect-mock": "^1.0"
     },
     "require-dev": {
         "moontoast/math": "^1.1",