ostree-sysroot: add debug option to help testing

ostree-sysroot: add debug option to help testing

If ostree is run in a test setup where it operates as root in a tmp
directory, it might cause issues to flag the deployments as immutable.
The test harness might simply be doing an `rm -rf` (effectively the case
for gnome-desktop-testing-runner), which will then fail.

We add a new debug option to the ostree_sysroot object using GLib's
GDebugKey functionality to allow our tests to communicate to ostree that
we don't want immutable deployments.