[chassis][Everflow] Add test coverage for inter-LC mirroring

[ysmanman]

Description

Current Everflow tests don't provide test coverage for chassis. Add new tests (or extend existing tests) to provide test coverage for chassis, e.g., inter-LC mirroring.

Steps to reproduce the issue:
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Describe the results you received:

Describe the results you expected:

Additional information you deem important:

**Output of `show version`:**

```
(paste your output here)
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**Attach debug file `sudo generate_dump`:**

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

[rlhui]

@ysmanman is taking this up

[rlhui]

@ysmanman , you have an update on this? Thanks.

[ysmanman]

The test gap was already fixed in #6945