Expand TabCompositionTests to cover all 4 sides and thicknesses 1, 2, 3#4859

Copilot wants to merge 3 commits intov2_developgui-cs/Terminal.Gui:v2_developfrom

copilot/expand-tabcompositiontests-coveragegui-cs/Terminal.Gui:copilot/expand-tabcompositiontests-coverageCopy head branch name to clipboard