MAINT: fix up tests that depend on the LoadError message to work.

We depend on require throwing about a specific file on disk, rather than just
generically, to ensure that we don't consume load errors from existing files.

This means our tests need to be updated to reflect that, by raising with
appropriate text when they stub require.

Reviewed-By: Pieter van de Bruggen <pieter@puppetlabs.com>