test: always copy custom_zodb.py, otherwise Python 3 incompatible one remains forever.

product/ERP5Type/tests/runUnitTest.py

@@ -214,9 +214,8 @@ def initializeInstanceHome(tests_framework_home,
         else:
           os.symlink(src, d)
     d = 'custom_zodb.py'
-    if not os.path.exists(d):
     src = os.path.join(tests_framework_home, d)
-      if os.path.islink(d):
+    if os.path.exists(d):
       os.remove(d)
     if WIN:
       shutil.copy(src, d)