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>
Overriding vfuncs can be useful, in particular when we convert
to ES modules, and exported symbols cannot easily be swapped out.
Adapt overrideMethod() to work correctly in that case as well.
Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/2809>
