Refactor test code structure

[SaxyPandaBear]

Currently, the test code layout is messy, with so many convoluted (and possibly circular) dependency directions for code execution. We should spend some time to clean up the codebase to make it easier to follow. My biggest concern is around the metric_value_benchmark test directory and everything in there. This is the biggest headache at the moment.

[khanhntd]

One of the reasons why I used the validator to generalize as much as possible (e.g for metric value benchmark, you can use the following agent config with the following validator config to validate all the things from metric_value_benchmark) and we don't need to rely on specific test case anymore)