As pointed out, above I thin execution config should not be written in some json file, specially not in the one
that is part of `.xpi` since that is single run configuration, which implies it's temporary & there for it's best to
store it in some temporary storage.

For now best compromise is for JPM was to set profile prefs to communicate execution config. For testing on places
where jpm isn't present I think the most reasonable option is to make `jpm.py` recreate same behavior of patching
passing config through prefs. For yet simpler cases like test packages I think it's reasonable to just add `prefs.js` file to a package with a run config.

For simple test packages I think we could also just have `index.js` that does `require("test")("./")` or something alike.