Add tests for PrimerProgressBar

[berkeenesakt]

This pull request adds tests for the PrimerProgressBar widget. The tests cover the following scenarios:

• PrimerProgressBar displays legend items for each segment
• PrimerProgressBar displays no legend items when segments list is empty
• PrimerProgressBar displays correct legend item when one segment has value of 0
• PrimerProgressBar displays legend ellipsis when legend items are longer than the maximum lines
• PrimerProgressBar displays correct progress bar value label with custom maximum value
• PrimerProgressBar displays correct progress bar color with custom SegmentedBarStyle

Please review and merge this pull request if it meets your requirements.

[fujidaiti]

Thank you! I'd like to add more tests before merging it.