Make sure to run pg_isready on correct port

The current code can have pg_isready unexpectedly succeed if there is a
server running on the default port. To avoid this we delay running the
test until after a node has been created but before it starts, and then
use that node's port, so we are fairly sure there is nothing running on
the port.

Backpatch to all live branches.

src/bin/scripts/t/080_pg_isready.pl

@@ -9,10 +9,11 @@ program_help_ok('pg_isready');
 program_version_ok('pg_isready');
 program_options_handling_ok('pg_isready');
 
-command_fails(['pg_isready'], 'fails with no server running');
-
 my $node = get_new_node('main');
 $node->init;
+
+$node->command_fails(['pg_isready'], 'fails with no server running');
+
 $node->start;
 
 $node->command_ok(