Add @need_network to test_no_error() in test_ignoreerrors.py

Needs network access for DNS:

warning No MX mail host for example.com found.

tests/checker/test_ignoreerrors.py

@@ -43,6 +43,7 @@ class TestFile(LinkCheckTest):
         ]
         self.direct(url, resultlines, confargs=confargs)
 
+    @need_network
     def test_no_error(self):
         """ Test that unmatched errors are not ignored. """
         self._test("mailto:good@example.com", "", "", True)