testsuite: Normalise trailing digits from hole fits output

The type variables in the holes fit output from
`abstract_refinement_hole_fits` is quite sensitive to compiler
configuration. Specifically, a slight change in the inlining
behavior of `throw` changes type variable naming in `(>>=)` and a few
others.

Ideally we would make hole fits output more deterministic but in the
meantime we simply normalise this difference away as it not relevant
to the test's goal.

(cherry picked from commit d029f170)