configitems: register the test 'ui.ioerrors' config

tests/test-rollback.t

@@ -220,10 +220,18 b' I/O errors on stdio are handled properly'
   > import errno
   > from mercurial.i18n import _
   > from mercurial import (
+  >     registrar,
   >     error,
   >     ui as uimod,
   > )
   > 
+  > configtable = {}
+  > configitem = registrar.configitem(configtable)
+  > 
+  > configitem('ui', 'ioerrors',
+  >     default=list,
+  > )
+  > 
   > def pretxncommit(ui, repo, **kwargs):
   >     ui.warn('warn during pretxncommit\n')
   > 
@@ -245,7 +253,7 b' I/O errors on stdio are handled properly'
   >         return getattr(self._o, attr)
   > 
   >     def write(self, msg):
-  >         errors = set(self._ui.configlist('ui', 'ioerrors', []))
+  >         errors = set(self._ui.configlist('ui', 'ioerrors'))
   >         pretxncommit = msg == 'warn during pretxncommit\n'
   >         pretxnclose = msg == 'warn during pretxnclose\n'
   >         txnclose = msg == 'warn during txnclose\n'