[test_gap] Validate the format of the packet once mirror configured

[kevinskwang]

Description

Need a new test case to validate the format of the packet once overflow configured, make sure there is no Vlan tag added in the GRE outer header.

Steps to reproduce the issue: 1. 2. 3.

Describe the results you received:

Describe the results you expected:

Additional information you deem important:

**Output of `show version`:**

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

```
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