of: overlay: unittest: Add test for unresolved symbol

Add a test to exercise the error paths when trying to apply an overlay
with an unresolved symbol and cleaning up the resulting partial state.

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Link: https://lore.kernel.org/r/580394587976975770c84411896fce9fbbcf25fa.1690533838.git.geert+renesas@glider.be
Signed-off-by: Rob Herring <robh@kernel.org>