text: Title: Migrate integration tests to slack_sdk package and add some legacy tests 
  Body: Currently, the integration tests still use `slack` package and verify if the WebClient and so on under the package works. As the source code is automatically generated from `slack_sdk.web.WebClient`, this is still good enough at this point. However, ideally we can use `slack_sdk` package for the tests and have additional tests for `slack` package.
  
  ### Category (place an `x` in each of the `[ ]`)
  
  - [ ] **slack_sdk.web.WebClient** (Web API client)
  - [ ] **slack_sdk.webhook.WebhookClient** (Incoming Webhook, response_url sender)
  - [ ] **slack_sdk.models** (UI component builders)
  - [ ] **slack_sdk.oauth** (OAuth Flow Utilities)
  - [ ] **slack_sdk.rtm.RTMClient** (RTM client)
  - [ ] **slack_sdk.signature** (Request Signature Verifier)
  - [x] tests
  
  ### Requirements
  
  Please read the [Contributing guidelines](https://github.com/slackapi/python-slack-sdk/blob/main/.github/contributing.md) and [Code of Conduct](https://slackhq.github.io/code-of-conduct) before creating this issue or pull request. By submitting, you are agreeing to those rules.
  