[test] increase simulator.go() time in key sequence jump test (#12249)

Increase the simulation wait time from 2 to 5 seconds in the
`test_mle_msg_key_seq_jump` after child restart.

This larger time window accounts for randomness in the timing of the
Child Update transmission. This makes the test more robust by ensuring
the child has sufficient time to send its "Child Update Request".

tests/scripts/thread-cert/test_mle_msg_key_seq_jump.py

@@ -256,7 +256,7 @@ class MleMsgKeySeqJump(thread_cert.TestCase):
         self.assertEqual(child.get_key_sequence_counter(), 25)
 
         child.start()
-        self.simulator.go(2)
+        self.simulator.go(5)
 
         self.assertEqual(child.get_state(), 'child')
         self.assertEqual(leader.get_key_sequence_counter(), 25)