tests: Fix check for global scope pollution

tests: Fix check for global scope pollution

Since commit c570011 dropped the `with` statement, this bit of
the test has only checked for writes to a specific variable
in the file.

There is no direct replacement for `with` here, the best we can
do (I think) is comparing property names on the global objects
before and after the `eval()` call.

Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/3132>