Solve cross-version-upgrade testing problem induced by 1fb57af9.

Renaming varchar_transform to varchar_support had a side effect
I hadn't foreseen: the core regression tests leave around a
transform object that relies on that function, so the name
change breaks cross-version upgrade tests, because the name
used in the older branches doesn't match.

Since the dependency on varchar_transform was chosen with the
aid of a dartboard anyway (it would surely not work as a
language transform support function), fix by just choosing
a different random builtin function with the right signature.
Also add some comments explaining why this isn't horribly unsafe.

I chose to make the same substitution in a couple of other
copied-and-pasted test cases, for consistency, though those
aren't directly contributing to the testing problem.

Per buildfarm.  Back-patch, else it doesn't fix the problem.