Move find_co_root into DBICTest::Util

Move find_co_root into DBICTest::Util

This is the first step of rearranging the utility pieces, removing reliance
on Path::Class in general

No visible functional changes, the old sub _find_co_root left as-is for the
time being, with an eval wrapped around it to retain the old "best effort"
behavior. Will be revisited in subsequent commits.