Unit Test for shiftDateRange

[boern99]

Breaking change

Proposed change

Type of change

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

Example configuration

@wendevlin: this is related to: #23228

It tests shiftDateRange function which is used in Energy-Dashboard / History and Logbook for Previous and Next-Buttons:
Tests for forward and backward: for a whole month and day

Additional information

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

Checklist

• The code change is tested and works locally.
• There is no commented out code in this PR.
• 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

[wendevlin]

Big thanks that you even made tests 🎉