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
shared libraries.

Since the implementation has this implicit assumption, mark the tests
as requiring dynway.

Test Plan: ./validate

Reviewers: bgamari

Reviewed By: bgamari

Subscribers: simonpj, rwbarton, carter

Differential Revision: https://phabricator.haskell.org/D5174