Evaluate options in sorted order to avoid spurious test failures.

src/zc/buildout/buildout.py

@@ -1083,7 +1083,7 @@ class Options(DictMixin):
             self._raw = self._do_extend_raw(name, self._raw, [])
 
         # force substitutions
-        for k, v in list(self._raw.items()):
+        for k, v in sorted(self._raw.items()):
             if '${' in v:
                 self._dosub(k, v)
 

src/zc/buildout/tests.py

@@ -137,10 +137,10 @@ It is an error to create a variable-reference cycle:
       Initializing.
       Getting section buildout.
       Initializing section buildout.
+      Getting option buildout:x.
       Getting option buildout:y.
       Getting option buildout:z.
       Getting option buildout:x.
-      Getting option buildout:y.
     Error: Circular reference in substitutions.
 
 It is an error to use funny characters in variable refereces: