Use --load-extension to set up for contrib/tcn's isolation tests.

Oversight in commit 418ec3207: it's better to do it like this,
else you have to drop and recreate the extension for each
permutation.  tcn.spec only has one permutation at present,
so this doesn't speed it up any, but it's still a bad example.

contrib/tcn/Makefile

@@ -7,6 +7,7 @@ DATA = tcn--1.0.sql
 PGFILEDESC = "tcn - trigger function notifying listeners"
 
 ISOLATION = tcn
+ISOLATION_OPTS = --load-extension=tcn
 
 ifdef USE_PGXS
 PG_CONFIG = pg_config

contrib/tcn/specs/tcn.spec

@@ -7,7 +7,6 @@
 
 setup
 {
-  CREATE EXTENSION tcn;
   CREATE TABLE mytable (key int PRIMARY KEY, value text);
   CREATE TRIGGER tcntrig AFTER INSERT OR UPDATE OR DELETE ON mytable
     FOR EACH ROW EXECUTE FUNCTION triggered_change_notification(mychannel);