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.c

@@ -153,6 +153,13 @@ ostree_sysroot_class_init (OstreeSysrootClass *klass)
 static void
 ostree_sysroot_init (OstreeSysroot *self)
 {
+  const GDebugKey keys[] = {
+    { "mutable-deployments", OSTREE_SYSROOT_DEBUG_MUTABLE_DEPLOYMENTS },
+  };
+
+  self->debug_flags = g_parse_debug_string (g_getenv("OSTREE_SYSROOT_DEBUG"),
+                                            keys, G_N_ELEMENTS (keys));
+
   self->sysroot_fd = -1;
   self->lock = (GLnxLockFile)GLNX_LOCK_FILE_INIT;
 }