There's no mechanism to follow dependencies for optional packages and packages used in testing.

Agreed. There's no reason to ship POD sanity and coverage tests, except to raise your Kwalitee and annoy your users. At least, there's no reason to run them as part of the default set of tests.

(I exaggerate; they might be valuable if you autogenerate your POD in such a way that it might end up invalid or missing on a different platform. I've never seen that though.)