It's based on old code from Antoine, see test_wrong_cert_tls12 and commit a6a4dc8.

I see. For test_wrong_cert_tls12(), SSLError or OSError is expected on connect(), not on read/write. There is a comment explaining the OSError:

# Expect either an SSL error about the server rejecting
# the connection, or a low-level connection reset (which
# sometimes happens on Windows)
s.connect((HOST, server.port))

test_wrong_cert_tls13() is different because TLS 1.3 is different. The test doesn't expect any error on connect(), only on read() or write(). It also explained in a comment:

# TLS 1.3 perform client cert exchange after handshake

In short, test_wrong_cert_tls12() looks correct, and I now understand how test_wrong_cert_tls13() inherited except OSError:. So I'm not convinced that it's ok to remove it :-)