Improve template test sensor

[sycx2]

Proposed change

Improves the template integration tests for the sensor platform. Rewrote tests to async pytest functions and using sensor.DOMAIN constant now instead of directly typed string "sensor".
Removed unnecessary duplicate of function test_available_template_with_entities.
Added domain as parameter to assert_setup_component.

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: Rewrite template unittest tests to pytest style test functions #40899
• 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.
• Untested files have been added to .coveragerc.

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[probot-home-assistant]

Hey there @PhracturedBlue, @tetienne, mind taking a look at this pull request as its been labeled with an integration (template) you are listed as a codeowner for? Thanks!
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[balloob]

Great! 🎉 🐬