Ease code before adding color and temperature to light template

[tetienne]

Breaking Change:

N/A

Description:

In order to ease the review, I will split https://github.com/home-assistant/home-assistant/pull/28141/files in several smaller PR.
This one add some tests and split async_update to avoid huge method.

Related issue (if applicable): N/A

Pull request with documentation for home-assistant.io (if applicable): N/A

Example entry for configuration.yaml (if applicable):

N/A

Checklist:

• The code change is tested and works locally.
• Local tests pass with tox. Your PR cannot be merged unless tests pass
• There is no commented out code in this PR.
• I have followed the development checklist

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

• Documentation added/updated in home-assistant.io

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

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

If the code does not interact with devices:

• Tests have been added to verify that the new code works.

[probot-home-assistant]

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