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.

src/libostree/ostree-sysroot-private.h

@@ -27,6 +27,13 @@
 
 G_BEGIN_DECLS
 
+typedef enum {
+
+  /* Don't flag deployments as immutable. */
+  OSTREE_SYSROOT_DEBUG_MUTABLE_DEPLOYMENTS = 1 << 0
+
+} OstreeSysrootDebugFlags;
+
 struct OstreeSysroot {
   GObject parent;
 
@@ -46,6 +53,7 @@ struct OstreeSysroot {
   /* Only access through ostree_sysroot_get_repo() */
   OstreeRepo *repo;
 
+  OstreeSysrootDebugFlags debug_flags;
 };
 
 #define OSTREE_SYSROOT_LOCKFILE "ostree/lock"