test: enhance testing and internal API robustness

[appleboy]

• Re-enable previously commented out test TestCommandWithIPv6 in plugin_test.go

[codecov[bot]]

Codecov Report

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

Project coverage is 38.44%. Comparing base (7d0a886) to head (1df34af).

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #269 +/- ## ======================================= Coverage 38.44% 38.44% ======================================= Files 2 2 Lines 476 476 ======================================= Hits 183 183 Misses 293 293 ```

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