add tests for `GetHorizontalFrameSize`

I don't think GetHorizontalFrameSize is working properly. I just ran an example that had a border and padding, but it only returned the amount of padding and didn't include the border

This is the program I ran https://github.com/charmbracelet/bubbletea/blob/master/examples/glamour/main.go

When I set glamour's word wrapping to vp.Width-4 it works fine, but if I use vp.Style.GetHorizontalFrameSize() it only returns 2, so breaks the table.