[scripts] fix test_router_reattach (#5517)

Fix test_router_attach.py failing by a small chance.

Root cause: routerdowngradethreshold restored back to default (23)
after node reset, so the test had 5/120 chance to downgrade. We should
set it back to 32 before starting it.

This commit also enhances the test for a little bit:
- Make sure the node reattached as Router within 1 second
- Make sure the node does not downgrade after Router Selection Jitter
  (3s)

tests/scripts/thread-cert/node.py

@@ -771,6 +771,10 @@ class Node:
         self.send_command(cmd)
         self._expect('Done')
 
+    def get_router_downgrade_threshold(self) -> int:
+        self.send_command('routerdowngradethreshold')
+        return int(self._expect_result(r'\d+'))
+
     def prefer_router_id(self, router_id):
         cmd = 'preferrouterid %d' % router_id
         self.send_command(cmd)