Add a way to include extra files on a per-test-binary and per-platform basis

[sunshowers]

There should be a way to say "I'd like to include some extra files" on specific platforms, or if specific packages are built. We can use most of the overrides framework we have already, but we need a way to restrict these overrides to only be per-test-binary, not per-test. That basically means allowing everything other than the test() predicate, I believe.

This will have to be done with some care.