The file listing needs to be sorted to return expectable results.

This seems like the only reasonable way to override, for example, grains.
Grains from `custom_grains_2` should override grains from `custom_grains_1`.

salt/loader.py

@@ -715,6 +715,7 @@ def grains(opts, force_refresh=False, proxy=None):
             # one parameter.  Then the grains can have access to the
             # proxymodule for retrieving information from the connected
             # device.
+            log.trace('Loading {0} grain'.format(key))
             if funcs[key].__code__.co_argcount == 1:
                 ret = funcs[key](proxy)
             else:
@@ -1179,7 +1180,8 @@ class LazyLoader(salt.utils.lazy.LazyDict):
         for mod_dir in self.module_dirs:
             files = []
             try:
-                files = os.listdir(mod_dir)
+                # Make sure we have a sorted listdir in order to have expectable override results
+                files = sorted(os.listdir(mod_dir))
             except OSError:
                 continue  # Next mod_dir
             for filename in files:

tests/integration/loader/test_ext_grains.py

@@ -32,6 +32,8 @@ class LoaderGrainsTest(ModuleCase):
     #    self.opts['grains'] = grains(self.opts)
 
     def test_grains_overwrite(self):
+        # Force a grains sync
+        self.run_function('saltutil.sync_grains')
         # To avoid a race condition on Windows, we need to make sure the
         # `test_custom_grain2.py` file is present in the _grains directory
         # before trying to get the grains. This test may execute before the