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.

tests/libtest.sh

@@ -22,6 +22,10 @@ test_tmpdir=$(pwd)
 
 export G_DEBUG=fatal-warnings
 
+# Don't flag deployments as immutable so that test harnesses can
+# easily clean up.
+export OSTREE_SYSROOT_DEBUG=mutable-deployments
+
 export TEST_GPG_KEYID_1="472CDAFA"
 export TEST_GPG_KEYID_2="CA950D41"
 export TEST_GPG_KEYID_3="DF444D67"