tests/unit: Access shell sources from resource

tests/unit: Access shell sources from resource

Unit tests currently load shell sources directly from the
filesystem.

This is currently blocking generated sources - namely config.js - to
ESM, because a relative import from the source dir will fail to
locate the file in the build dir.

Address this by using the same GResource as gnome-shell instead of
direct filesystem access, as the resource will always include all
sources files at the expected location.

Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/2880>