Arg increfing test.

tests/run/arg_incref.pyx

+def f(dict d):
+    """
+    >>> f({1:1, 2:2})
+    [1, 2]
+    """
+    cdef dict d_new = {}
+    l = []
+    for k in d:
+        d = d_new
+        l.append(k)
+    l.sort()
+    return l