of: unittest: Cleanup partially-applied overlays

When of_overlay_fdt_apply() fails, the changeset may be partially
applied, and the caller is still expected to call of_overlay_remove() to
clean up this partial state.  However, overlay_17 is the only test that
takes care of cleaning up after an (expected) failure.

Instead of adding cleanup code to each individual test, extend
overlay_info with the optional expected return value of
of_overlay_remove(), and handle cleanup in the overlay_data_apply()
helper.  While at it, simplify the end marker in the overlay_info table.

Update the expected error output for errors during the newly cleanup.

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Link: https://lore.kernel.org/r/594a6a8934e5569bf96d317a6a3c0a9129a2ae20.1690533838.git.geert+renesas@glider.be
[robh: update and fix EXPECT messages]
Signed-off-by: Rob Herring <robh@kernel.org>