ERP5Type.Core.Folder: Simplify proxy methods.

Also, clarify what exactly was the broken BTree marker value, fixing
isBTree accessor. Update associated tests.

product/ERP5Type/tests/testFolderMigration.py

@@ -443,10 +443,9 @@ class TestFolderMigration(ERP5TypeTestCase, LogInterceptor):
       self.assertEqual(self.folder.isBTree(), True)
       self.assertEqual(self.folder.isHBTree(), False)
 
-      setattr(self.folder,'_folder_handler','VeryWrongHandler')
+      self.folder._folder_handler = 'CMFBTreeFolderHandler'
       self.tic()
 
-      self.assertEqual(self.folder.isBTree(), False)
       self.assertEqual(self.folder.isHBTree(), False)
 
       self.assertEqual(self.folder._fixFolderHandler(), True)
@@ -465,10 +464,9 @@ class TestFolderMigration(ERP5TypeTestCase, LogInterceptor):
       self.assertEqual(self.folder.isBTree(), True)
       self.assertEqual(self.folder.isHBTree(), False)
 
-      setattr(self.folder,'_folder_handler','VeryWrongHandler')
+      self.folder._folder_handler = 'CMFBTreeFolderHandler'
       self.tic()
 
-      self.assertEqual(self.folder.isBTree(), False)
       self.assertEqual(self.folder.isHBTree(), False)
 
       self.folder.migrateToHBTree()