At test-time, add `PATHS_JSON`, `INDEX_JSON` paths

[wolfv]

It might be useful to do some checks on the JSON files for e.g. paths.json to validate that files are there. However, it's hard to find this file (one would have to load the PrefixRecord from conda-meta and then find from where the package was linked).

We could add in some env vars to make it easier for test scripts to find these files.