Some `ImagePartialUpdateTest` test are calling code that needs access to
a valid `G_MAIN`. So store the generated main there as part of the setup
step, and reset G_MAIN to its original value (should be NULL) in the
teardown step.
NOTE: Things like `ID_BLEND_PATH_FROM_GLOBAL` and
`BKE_main_blendfile_path_from_global` are pure evil. It may be necessary
in a very few small cases, but their current usages need a lot of strong
cleanup.
Bastien Montagne