Move cast config flow tests to test_config_flow

[emontnemery]

Proposed change

Move cast config flow tests to test_config_flow as suggested here: https://github.com/home-assistant/core/pull/47269/files#pullrequestreview-620835556

Type of change

• Dependency upgrade
• Bugfix (non-breaking change which fixes an issue)
• New integration (thank you!)
• New feature (which adds functionality to an existing integration)
• Breaking change (fix/feature causing existing functionality to break)
• Code quality improvements to existing code or addition of tests

Additional information

• This PR is related to issue: Add allowed UUIDs and ignore CEC to Google Cast options flow #47269

Checklist

• The code change is tested and works locally.
• Local tests pass. Your PR cannot be merged unless tests pass
• There is no commented out code in this PR.
• I have followed the development checklist
• The code has been formatted using Black (black --fast homeassistant tests)
• Tests have been added to verify that the new code works.

If user exposed functionality or configuration variables are added/changed:

• Documentation added/updated for www.home-assistant.io

If the code communicates with devices, web services, or third-party tools:

• The manifest file has all fields filled out correctly.
  Updated and included derived files by running: python3 -m script.hassfest.
• New or updated dependencies have been added to requirements_all.txt.
  Updated by running python3 -m script.gen_requirements_all.
• Untested files have been added to .coveragerc.

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To help with the load of incoming pull requests:

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[MartinHjelmare]

The tests that are not testing the result of a flow should stay in the test_init.py module, eg test_import.

[MartinHjelmare]

Thanks!

[emontnemery]

Tagging for 2021.4 to allow cherry-picking #49051