Make subscription collation test work independent of locale

We need to set the database to UTF8 encoding so that the test can use
Unicode escapes.

src/test/subscription/t/012_collation.pl

@@ -16,11 +16,11 @@ else
 }
 
 my $node_publisher = get_new_node('publisher');
-$node_publisher->init(allows_streaming => 'logical');
+$node_publisher->init(allows_streaming => 'logical', extra => [ '--locale=C', '--encoding=UTF8' ]);
 $node_publisher->start;
 
 my $node_subscriber = get_new_node('subscriber');
-$node_subscriber->init(allows_streaming => 'logical');
+$node_subscriber->init(allows_streaming => 'logical', extra => [ '--locale=C', '--encoding=UTF8' ]);
 $node_subscriber->start;
 
 my $publisher_connstr = $node_publisher->connstr . ' dbname=postgres';