Make FakeSwiftCall.DUMMY_VALUE a bespoke singleton

In test assertion failure messages the DUMMY_VALUE would be shown as
<object object ...> which is not immediately recognisable as the dummy
value..

This change makes the DUMMY_VALUE show as
<test.unit.common.middleware.helpers.FakeSwiftDummyValue object ...>.

Change-Id: I73fac245c65c48d45e4919c6f15688d482148407
Related-Change: I332ce724aa10287800cbec8ca21aacc3bbd3c22a

test/unit/common/middleware/helpers.py

@@ -51,11 +51,21 @@ class LeakTrackingIter(object):
         self.mark_closed(self.key)
 
 
+class FakeSwiftDummyValue:
+    _instance = None
+
+    @classmethod
+    def singleton(cls):
+        if not cls._instance:
+            cls._instance = FakeSwiftDummyValue()
+        return cls._instance
+
+
 class FakeSwiftCall(object):
     """
     Encapsulate properties of a request captured by FakeSwift.
     """
-    DUMMY_VALUE = object()
+    DUMMY_VALUE = FakeSwiftDummyValue.singleton()
 
     def __init__(self, req):
         self.req = req