Add test for CombinedArtifactCache's behavior

Makes sure that remote + local correctly "backfills"
local when fetching from remote.

This is just documenting/asserting current behavior.

Testing Done:
https://travis-ci.org/pantsbuild/pants/builds/38635762

Reviewed at https://rbcommons.com/s/twitter/r/982/
Sync of https://github.com/pantsbuild/pants/commit/dc00b02a05e41af1c5a05c9598264c1d27980053