Improve nice_relative_time tests

- Add more variations
- Remove import from mycroft-core
- Remove reference to local time to ensure deterministic outcome

Ensure relative datetimes are compared in the same timezone.

All comparisons are made in UTC to prevent issues of the two datetimes
passed in having different timezone info.

Remove unnecessary duplicate function

The nice_relative_time function is already defined in the top level format.py
hence it isn't needed in format_en.py.

This convention may change in the future.

Added general nice_relative_time_en

Added test for function nice_relative_time

Added function nice_relative_time (used in skill-alarm but not localized)

Co-authored-by: coderabbitai[bot] <136622811+coderabbitai[bot]@users.noreply.github.com>

Co-authored-by: coderabbitai[bot] <136622811+coderabbitai[bot]@users.noreply.github.com>