Fixes https://github.com/flutter/flutter/issues/129969 by making tooltip text unselectable (for now). 
Also fixes some other issues uncovered when I was writing the tests.

Currently `getTransformTo` only works on ancestors. I'll try to add a new method that computes the transform from 2 arbitrary render objects in the same render tree in a follow-up PR and make `Selectable` use that method instead.