Nut: Use coordinator data, code cleanup and add test coverage

[ollo69]

Proposed change

I did some code clean-up for Nut integration, removing useless code and improving tests. To simplify approval process I will split changes in 2 or 3 PR.
On this PR:

• Implemented use of coordinator.data to store and fetch Nut status
• Removed unreachable or useless codes
• Added tests for sensor module (removed exclusion from .coveragerc)

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 fixes or closes issue: fixes #
• This PR is related to issue:
• Link to documentation pull request:

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.
• For the updated dependencies - a link to the changelog, or at minimum a diff between library versions is added to the PR description.
• Untested files have been added to .coveragerc.

The integration reached or maintains the following Integration Quality Scale:

• No score or internal
• 🥈 Silver
• 🥇 Gold
• 🏆 Platinum

To help with the load of incoming pull requests:

• I have reviewed two other open pull requests in this repository.

[probot-home-assistant]

Hey there @bdraco, mind taking a look at this pull request as it has been labeled with an integration (nut) you are listed as a code owner for? Thanks!
_{^{(message by CodeOwnersMention)}}

[bdraco]

Thank you for your contribution thus far! 🎖 Since this is a significant contribution, we would appreciate you'd added yourself to the list of code owners for this integration. ❤️

Please, add your GitHub username to the manifest.json of this integration.

For more information about "code owners", see: Architecture Decision Record 0008: Code owners.