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>
Commit 46874eed0 accidentally changed the behavior of the function in
an incompatible way. Before addressing the actual issue, add a reproducer
to the unit tests to hopefully prevent future breakage.
https://gitlab.gnome.org/GNOME/gnome-shell/merge_requests/615
