Add support for "days" unit for STESTI sensor in APCUPSD integration

[yuxincs]

Breaking change

sensor.ups_self_test_interval sensor now properly splits unit "days" from the raw data. Previously, it will show 7 days as sensor state, now it shows 7 with a unit UnitOfTime.DAYS.

Proposed change

Although APCUPSD manual suggests that STESTI field will report the interval in hours, we have reports from users that it can actually contain numeric values with units days, e.g., 7 days.
#93132 (comment)
#81476 (comment)

This PR adds support for days unit and properly splits the unit from the raw strings instead. Also, this PR removes the state class of this sensor since it is not total_increasing, it is actually a user-configurable option.

Tests for STESTI have been added to prevent future regressions.

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)
• Deprecation (breaking change to happen in the future)
• 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 apcupsd: multiple sensors no longer reporting data #93132
• This PR is related to issue: apcupsd: multiple sensors no longer reporting data #93132
• 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
• I have followed the perfect PR recommendations
• 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.

To help with the load of incoming pull requests:

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

[emontnemery]

LGTM, thanks @yuxincs