catch malformed cert warning in tests

we may need to catch this properly in `get_cert` at some point,
let's see if this ever turns out to be an issue.

test/mitmproxy/addons/test_tlsconfig.py

@@ -86,7 +86,8 @@ class TestTlsConfig:
 
             with open(tdata.path("mitmproxy/data/invalid-subject.pem"), "rb") as f:
                 ctx.server.certificate_list = [certs.Cert.from_pem(f.read())]
-            assert ta.get_cert(ctx)  # does not raise
+            with pytest.warns(UserWarning, match="Country names should be two characters"):
+                assert ta.get_cert(ctx)  # does not raise
 
     def test_tls_clienthello(self):
         # only really testing for coverage here, there's no point in mirroring the individual conditions