Fix test_layer_appends to work when build directory is not inside
COREBASE.
Fixes [YOCTO #8639].
(From OE-Core master rev: 0f146e77655d153d3f9a59e489265450f08c6ad7)
(From OE-Core rev: e353b303e271368426e71810bb75173ca6f53455)
Signed-off-by: Paul Eggleton <paul.eggleton@linux.intel.com>
Signed-off-by: Ross Burton <ross.burton@intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>