This commit is summarized as:
- Add the struct `TestConfig` as a parameter for `app.Test()` instead of `timeout`
- Add documentation of `TestConfig` to docs/api/app.md and in-line
- Modify middleware to use `TestConfig` instead of the previous implementation

Fixes gofiber#3149

- Fixes Test_Utils_TestConn_Closed_Write
- Fixes missing regular write test

- Rename TestConfig.ErrOnTimeout to TestConfig.FailOnTimeout
- Update documentation to use changed name
  - Also fix stale documentation about passing Timeout as a
    single argument