Fix plugin tests requirements
Unfortunately the implementation has confused the ability to make
dynamic libraries with dynamic way.
This constraint is only true for systems that require -fPIC for
Since the implementation has this implicit assumption, mark the tests
as requiring dynway.
Test Plan: ./validate
Reviewed By: bgamari
Subscribers: simonpj, rwbarton, carter
Differential Revision: https://phabricator.haskell.org/D5174