SSL async callback: cert is not always from mbedtls_ssl_conf_own_cert

The certificate passed to async callbacks may not be the one set by
mbedtls_ssl_conf_own_cert. For example, when using an SNI callback,
it's whatever the callback is using. Document this, and add a test
case (and code sample) with SNI.

tests/ssl-opt.sh

@@ -4112,6 +4112,20 @@ run_test    "SSL async private: sign, RSA, TLS 1.1" \
             -s "Async sign callback: using key slot " \
             -s "Async resume (slot [0-9]): sign done, status=0"
 
+requires_config_enabled MBEDTLS_SSL_ASYNC_PRIVATE
+run_test    "SSL async private: sign, SNI" \
+            "$P_SRV debug_level=3 \
+             async_operations=s async_private_delay1=0 async_private_delay2=0 \
+             crt_file=data_files/server5.crt key_file=data_files/server5.key \
+             sni=localhost,data_files/server2.crt,data_files/server2.key,-,-,-,polarssl.example,data_files/server1-nospace.crt,data_files/server1.key,-,-,-" \
+            "$P_CLI server_name=polarssl.example" \
+            0 \
+            -s "Async sign callback: using key slot " \
+            -s "Async resume (slot [0-9]): sign done, status=0" \
+            -s "parse ServerName extension" \
+            -c "issuer name *: C=NL, O=PolarSSL, CN=PolarSSL Test CA" \
+            -c "subject name *: C=NL, O=PolarSSL, CN=polarssl.example"
+
 requires_config_enabled MBEDTLS_SSL_ASYNC_PRIVATE
 run_test    "SSL async private: decrypt, delay=0" \
             "$P_SRV \