[css-anchor-position-1] position-try-options needs to set auto insets to zero for sizing

[fantasai]

For each entry in the position options list, apply that position option to the box, and find the specified inset-modified containing block size that results from those styles. Stably sort the position options list according to this size, with the largest coming first.'

There's a problem here with auto insets: they would require doing full layout of the box into that position to find the IMCB. But we don't really want to do that for this, so we should assume that auto insets are zero for this purpose.