[dualtor][test gap] PXEboot scenario

[zjswhhh]

Description

Steps to reproduce the issue:

1. shutdown icmp_responder
2. reboot server

Describe the results you expected:
Scenario 1 link flap after boot up.
One side ToR toggles mux, and the other side is able to sync up in short amount of time.
Scenario 2 start icmp_responder after boot up
Both ToRs get back to healthy state immediately.

Additional information you deem important:

**Output of `show version`:**

```
(paste your output here)
```

**Attach debug file `sudo generate_dump`:**

```
(paste your output here)
```