regenerate the testdata certificate with SAN instead of CommonName

As required by Go 1.15.

internal/testdata/generate_key.sh

@@ -0,0 +1,24 @@
+#!/bin/bash
+
+set -e
+
+echo "Generating CA key and certificate:"
+openssl req -x509 -sha256 -nodes -days 3650 -newkey rsa:2048 \
+  -keyout ca.key -out ca.pem \
+  -subj "/O=quic-go Certificate Authority/"
+
+echo "Generating CSR"
+openssl req -out cert.csr -new -newkey rsa:2048 -nodes -keyout priv.key \
+  -subj "/O=quic-go/"
+
+echo "Sign certificate:"
+openssl x509 -req -sha256 -days 3650 -in cert.csr  -out cert.pem \
+  -CA ca.pem -CAkey ca.key -CAcreateserial \
+  -extfile <(printf "subjectAltName=DNS:localhost")
+
+# debug output the certificate
+openssl x509 -noout -text -in cert.pem
+
+# we don't need the CA key, the serial number and the CSR any more
+rm ca.key cert.csr ca.srl
+