CPP macro for --enable-testing

I want to conditionally export certain parts like constructors, types or instances which are needed to write test cases. Otherwise I wouldn't want them to be exposed to the library user.

The approach I am following now is to have `Foo.Internal` export necessary elements.

Start by tracing how Cabal handles --enable-testing and CPP-related configuration. The issue names no source file or test, so first locate the relevant build and test entry points, then compare the requested conditional exports with the current Foo.Internal approach. Done should include an agreed macro behavior and verification that test-only exports remain hidden from normal library users.

Written by the indexing model from the issue text.