Make test-read-dwarf.cc and test-read-write.cc abidiff the ABIs

Make test-read-dwarf.cc and test-read-write.cc abidiff the ABIs

use abidiff to compare the ABI of the input ABI against the result of
writing that input back into an XML format.  It should yield the empty
set.  I wonder why I haven't done this since the beginning.  It turned
out we had to fix many things to make it work now.  Just using "GNU
diff" to compare the output against a reference output is definitely
not enough.

	* tests/test-read-dwarf.cc (main): Use abidiff to compare the
	input elf file with the XML emitted.  That should yield the empty
	set.
	* tests/test-read-write.cc (main): Likewise, use abidiff to
	compare the input abixml file with the one that is emitted.

Signed-off-by: Dodji Seketeli <dodji@redhat.com>