fix & docs & test: 修复全局数据库配置无法生效的问题以及新增测试用例，修改文档说明，新增reader/writer注册函… …数测试用例

[codecov-commenter]

Codecov Report

Patch coverage: 100.00% and project coverage change: -0.06 :warning:

Comparison is base (9e41ed7) 92.39% compared to head (9209639) 92.34%.

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Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #18 +/- ## ========================================== - Coverage 92.39% 92.34% -0.06% ========================================== Files 121 123 +2 Lines 6591 6635 +44 ========================================== + Hits 6090 6127 +37 - Misses 399 404 +5 - Partials 102 104 +2 ``` | [Impacted Files](https://app.codecov.io/gh/Breeze0806/go-etl/pull/18?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=None) | Coverage Δ | | |---|---|---| | [datax/core/job/container.go](https://app.codecov.io/gh/Breeze0806/go-etl/pull/18?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=None#diff-ZGF0YXgvY29yZS9qb2IvY29udGFpbmVyLmdv) | `92.04% <100.00%> (+0.15%)` | :arrow_up: | | [datax/plugin/writer/dbms/config.go](https://app.codecov.io/gh/Breeze0806/go-etl/pull/18?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=None#diff-ZGF0YXgvcGx1Z2luL3dyaXRlci9kYm1zL2NvbmZpZy5nbw==) | `100.00% <100.00%> (ø)` | | ... and [5 files with indirect coverage changes](https://app.codecov.io/gh/Breeze0806/go-etl/pull/18/indirect-changes?src=pr&el=tree-more&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=None)

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