Remove checkLen() from BasicStorage and add it to LocalStorage.

src/ZODB/tests/BasicStorage.py

@@ -68,14 +68,6 @@ class BasicStorage:
         # Finish the transaction.
         assert newrevid <> revid
 
-    def checkLen(self):
-        # The length of the database ought to grow by one each time
-        assert len(self._storage) == 0
-        self._dostore()
-        assert len(self._storage) == 1
-        self._dostore()
-        assert len(self._storage) == 2
-
     def checkNonVersionStoreAndLoad(self):
         oid = self._storage.new_oid()
         self._dostore(oid=oid, data=7)

src/ZODB/tests/LocalStorage.py

+class LocalStorage:
+    """A single test that only make sense for local storages.
+
+    A local storage is one that doens't use ZEO. The __len__()
+    implementation for ZEO is inexact.
+    """
+    def checkLen(self):
+        # The length of the database ought to grow by one each time
+        assert len(self._storage) == 0
+        self._dostore()
+        assert len(self._storage) == 1
+        self._dostore()
+        assert len(self._storage) == 2
+