Re-enable butler test lost due to missing function definition.

...and fix a pretty obvious bug in it.

Looks like a bad conflict resolution, but that wouldn't explain the bug.

In any case, I'm quite surprised flake8 wasn't mad about the
unqualified return statement in the middle of a function.

tests/test_butler.py

@@ -291,6 +291,7 @@ def runPutGetTest(self, storageClass, datasetTypeName):
 
         return butler
 
+    def testDeferredCollectionPassing(self):
         # Construct a butler with no run or collection, but make it writeable.
         butler = Butler(self.tmpConfigFile, writeable=True)
         # Create and register a DatasetType
@@ -331,7 +332,7 @@ def runPutGetTest(self, storageClass, datasetTypeName):
         # Deleting the dataset from the new collection should make it findable
         # in the original collection.
         butler.prune([ref], collection="tagged")
-        self.assertFalse(butler.datasetExists(datasetType, dataId, collection=run))
+        self.assertTrue(butler.datasetExists(datasetType, dataId, collection=run))
 
 
 class ButlerTests(ButlerPutGetTests):