auto: fix big program issue in quickcheck

Resizing wasn't aggressive enough in for AST arbitrary instances.
Trees are exponential, and `n - 1` is linear. Replace the resize
function with:

  n / (1 + Random.int n)

to increase the likelihood that ASTs will get smaller the deeper down in
the data structure an arbitrary instance is called.
(cherry picked from commit 6f3bfbe01718d9a29c8d2498e23729ac3a17e38b)

Conflicts:
	quickcheck/NetKAT_Arbitrary.ml