[ruby/openssl] Relax error message check for OpenSSL 3.1

A tentative measures fo ruby/openssl#606.

With OpenSSL 3.1.0, the error message at connection using "self-signed
certificate" seems to return `SSL_R_TLSV1_ALERT_UNKNOWN_CA` instead of
`SSL_R_CERTIFICATE_VERIFY_FAILED`.

ruby/openssl@fc4629d246

test/openssl/test_ssl.rb

@@ -1046,9 +1046,7 @@ def test_connect_certificate_verify_failed_exception_message
     start_server(ignore_listener_error: true) { |port|
       ctx = OpenSSL::SSL::SSLContext.new
       ctx.set_params
-      # OpenSSL <= 1.1.0: "self signed certificate in certificate chain"
-      # OpenSSL >= 3.0.0: "self-signed certificate in certificate chain"
-      assert_raise_with_message(OpenSSL::SSL::SSLError, /self.signed/) {
+      assert_raise_with_message(OpenSSL::SSL::SSLError, /certificate/) {
         server_connect(port, ctx)
       }
     }