The regression for `PresetTree.write` lived two layers up, so this layer's own
assertion could not fail: it checked the file after an ordinary teardown, and
the Loader's unload listener returns early when the whole tree is being
disposed, so the override never ran. Moves the self-disposing-row test down to
the layer that adds the override, in a temp root so a committed fixture cannot
be damaged by the run that proves the bug.
6435463edc