lib/zodb: Mark test_zconn_at as xfail on plain ZODB4

This way on plain ZODB4 the following non-wcfs tests will continue to
pass

   test.py/fs-!wcfs
   test.py/zeo-!wcfs
   test.py/neo-!wcfs

instead of failing as e.g. in here:

   https://nexedijs.erp5.net/#/test_result_module/20211116-123A66706

On plain ZODB4 WCFS-related functionality - which uses zconn_at - will
continue to raise corresponding assertion in WCFS-related tests, as e.g. in

https://nexedijs.erp5.net/#/test_result_module/20211116-123A66706/6

lib/tests/test_zodb.py

@@ -17,7 +17,7 @@
 #
 # See COPYING file for full licensing terms.
 # See https://www.nexedi.com/licensing for rationale and options.
-from wendelin.lib.zodb import LivePersistent, deactivate_btree, dbclose, zconn_at, zstor_2zurl
+from wendelin.lib.zodb import LivePersistent, deactivate_btree, dbclose, zconn_at, zstor_2zurl, zmajor, _zhasNXDPatch
 from wendelin.lib.testing import getTestDB
 from wendelin.lib import testing
 from persistent import Persistent, UPTODATE, GHOST, CHANGED
@@ -238,6 +238,9 @@ def test_deactivate_btree():
 # verify that zconn_at gives correct answer.
 @func
 def test_zconn_at():
+    if zmajor == 4 and not _zhasNXDPatch('conn:MVCC-via-loadBefore-only'):
+        pytest.xfail(reason="zconn_at needs https://lab.nexedi.com/nexedi/ZODB/merge_requests/1 to work on ZODB4")
+
     stor = testdb.getZODBStorage()
     defer(stor.close)
     db  = DB(stor)

lib/zodb.py

@@ -182,12 +182,17 @@ def before2at(before): # -> at
 
 # _zassertHasNXDPatch asserts that ZODB is patched with specified Nexedi-provided patch.
 def _zassertHasNXDPatch(patch, details_link):
-    nxd_patches = getattr(ZODB, 'nxd_patches', set())
-    if patch not in nxd_patches:
+    if not _zhasNXDPatch(patch):
         raise AssertionError(
             "ZODB%s is not patched with required Nexedi patch %r\n\tSee %s for details" %
             (zmajor, patch, details_link))
 
+# _zhasNXDPatch returns whether ZODB is patched with specified Nexedi-provided patch.
+def _zhasNXDPatch(patch):
+    nxd_patches = getattr(ZODB, 'nxd_patches', set())
+    return (patch in nxd_patches)
+
+
 # _zversion returns ZODB version object
 def _zversion():
     dzodb3 = pkg_resources.working_set.find(pkg_resources.Requirement.parse('ZODB3'))