of: unittest: Merge of_unittest_apply{,_revert}_overlay_check()

of_unittest_apply_overlay_check() and the first part of
of_unittest_apply_revert_overlay_check() are identical.
Reduce code duplication by replacing them by two wrappers around a
common helper.

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Link: https://lore.kernel.org/r/4bafe338655beab6b244047e737c0c2aa60b613d.1690533838.git.geert+renesas@glider.be
Signed-off-by: Rob Herring <robh@kernel.org>