of: unittest: Add separators to of_unittest_overlay_high_level()

It is hard to see the start and end of each individual test in
of_unittest_overlay_high_level().  Add visual cues in the form of
separator comments, like was done in of_unittest_overlay_notify().

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Link: https://lore.kernel.org/r/84530100a54a1fac932008057746015e65bc4d9a.1690533838.git.geert+renesas@glider.be
Signed-off-by: Rob Herring <robh@kernel.org>