Enhancement Request: Allow usage of unbundled resources in java and scala tests

Created by: samsieber

I'd like to request that Buck be enhanced to allow for usage of unbundled resources in java and scala tests I'd like to migrate a fairly large gradle project (30+ sub-projects, 2000+ tests) to Buck, however, many of our tests rely on the directory from the getResource() call in java to return a local file reference, not a jar.

So for a specific implementation idea, something like external_resources and external_resource_root would be great; they would for resources to be accessible by tests, but located in the file system, not the jar.