add chained socks5 proxy testcase

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 41.11%. Comparing base (ecb3325) to head (009c2ff).

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #368 +/- ## ========================================== + Coverage 40.43% 41.11% +0.67% ========================================== Files 1089 1089 Lines 98366 98360 -6 ========================================== + Hits 39778 40440 +662 + Misses 58588 57920 -668 ``` | [Flag](https://app.codecov.io/gh/bytedance/g3/pull/368/flags?src=pr&el=flags&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=bytedance) | Coverage Δ | | |---|---|---| | [g3proxy](https://app.codecov.io/gh/bytedance/g3/pull/368/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=bytedance) | `41.11% <ø> (+0.67%)` | :arrow_up: | | [lib](https://app.codecov.io/gh/bytedance/g3/pull/368/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=bytedance) | `?` | | Flags with carried forward coverage won't be shown. [Click here](https://docs.codecov.io/docs/carryforward-flags?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=bytedance#carryforward-flags-in-the-pull-request-comment) to find out more.

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