Add and refactor tests for features of element "buttongroups" on a test-site

[MayaElf]

Add and refactor tests for features of element "buttongroups" on a test-site

• [x] form(shaped)
• [x] dense
• [x] group
• [x] borderless
• [x] theme
• [x] background-color
• [x] tile
• [x] icon

[MayaElf]

blocked by #4182 (when element 'shaped' is added to test-site than ButtonGroupsTests should be refactored)