Make new SSL TAP test for channel_binding more robust

The test would fail in an environment including a certificate file in
~/.postgresql/.  bdd6e9b fixed a similar failure, and d6e612f introduced
the same problem again with a new test.

Author: Kyotaro Horiguchi
Discussion: https://postgr.es/m/20200804.120033.31225582282178001.horikyota.ntt@gmail.com
Backpatch-through: 13

src/test/ssl/t/002_scram.pl

@@ -97,7 +97,7 @@ my $client_tmp_key = "ssl/client_scram_tmp.key";
 copy("ssl/client.key", $client_tmp_key);
 chmod 0600, $client_tmp_key;
 test_connect_fails(
-	"sslcert=ssl/client.crt sslkey=$client_tmp_key hostaddr=$SERVERHOSTADDR",
+	"sslcert=ssl/client.crt sslkey=$client_tmp_key sslrootcert=invalid hostaddr=$SERVERHOSTADDR",
 	"dbname=certdb user=ssltestuser channel_binding=require",
 	qr/channel binding required, but server authenticated client without channel binding/,
 	"Cert authentication and channel_binding=require");