of: unittest: Improve messages and comments in apply/revert checks

Miscellaneous improvements for the apply and apply/revert checks,
making them more similar:
  - Fix inverted comment for before state check,
  - Add more comments to improve symmetry,
  - Fix grammar s/must be to set to/must be in/,
  - Avoid saying "create" in messages, as the actual operation depends
    on the value of the before/after parameters.

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Link: https://lore.kernel.org/r/326ecfe0889c53d2cfff31b3bf950d0b70be225f.1690533838.git.geert+renesas@glider.be


Signed-off-by: Rob Herring <robh@kernel.org>