Fix boneheaded regression test.

Using 'random' in a test exercising a known bug is a code stench.

BTrees/tests/testBTrees.py

@@ -313,7 +313,9 @@ class BugFixes(unittest.TestCase):
                 t[id] = (id, ToBeDeleted(id), _u('somename'))
             else:
                 #del
-                id = trandom.choice(list(ids.keys()))
+                keys = list(ids.keys())
+                if keys:
+                    id = trandom.choice(keys)
                     del t[id]
                     del ids[id]