of: convert unittest overlay devicetree source to sugar syntax

The unittest-data overlays have been pulled into proper overlay
devicetree source files without changing their format.  The
next step is to convert them to use sugar syntax instead of
hand coding overlay fragments structure.

A few of the overlays can not be converted because they test
absolute target paths in the overlay fragment.  dtc does not
generate this type of target:
  overlay_0.dts
  overlay_1.dts
  overlay_12.dts
  overlay_13.dts

Two pre-existing unittest overlay devicetree source files are
also converted:
  overlay_bad_phandle.dts
  overlay_bad_symbol.dts

Signed-off-by: Frank Rowand <frank.rowand@sony.com>