of: unittest: change references to obsolete overlay id

Unittest inconsistently interchanges overlay changeset id and
overlay id.  Change variable names of overlay id to overlay
changeset id.

Do not fix variable names in the overlay tracking functions
of_unittest_overlay_tracked(), of_unittest_track_overlay(), and
of_unittest_destroy_tracked_overlays() which will be replaced in
a following commit.

Signed-off-by: Frank Rowand <frank.rowand@sony.com>
Signed-off-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/20220101033329.1277779-2-frowand.list@gmail.com