of: unittest: re-implement overlay tracking

Some overlays are tracked when they are applied.  The tracked overlays
are later removed after the overlay tests are completed.  The old
implementation makes assumptions about the expected values for
overlay changeset id created by the overlay apply which result
in fragile code.  The new code removes the assumptions.

A symptom that exposes a problem with the tracking code is a
warning "UBSAN: shift-out-of-bounds in drivers/of/unittest.c:1933:36",
Kernel Version: 5.15-rc7, PPC-64, Talos II.  This results from variable
"id" value of -1 in the final line of of_unittest_untrack_overlay().

Reported-by:  <erhard_f@mailbox.org>
Signed-off-by: Frank Rowand <frank.rowand@sony.com>
Signed-off-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/20220101033329.1277779-3-frowand.list@gmail.com