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>
Removes the init() function in favor of executing all environment
changes when the file is imported.
Additionally ports all unit tests using imports.gi.environment.init() to
use the updated module.
Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/2822>
