replace fuzzing wrapper with a shell script

This is part of a series of changes to deprecate Python 2, but it has some other
notable effects as well:

  1. We no longer do any env tweaking as rumur is now in our $PATH during
     testing.
  2. We use the travis_wait wrapper to sustain execution rather than our own
     invisible character output.
  3. We use timeout rather than our own logic. This restricts fuzzing to
     Linux-only, but we were only using it on Linux anyway.

Github: related to #167 "switch all Python dependencies to Python 3 in
  packaging/debian"
Github: related to #170 "optimise testing"