Tunable address, Ubuntu 16.04

Make the test for #11108 less fragile

This change should close #11108 by fixing the test case.

This commit fixes two issues:

* Make sure that each weak pointer we allocate has a constructor as the
  key, not a thunk. A failure to do so meant these weak pointers died
  prematurely on the 'ghci' WAY.

* Don't print anything in the finalizer, because they are not guaranteed
  to run.

Test Plan: validate

Reviewers: austin, simonmar, erikd, bgamari

Reviewed By: erikd, bgamari

Subscribers: thomie

Differential Revision: https://phabricator.haskell.org/D2512

GHC Trac Issues: #11108