Use abidw --abidiff in test-read-dwarf.cc

Use abidw --abidiff in test-read-dwarf.cc

That test was doing several sub-tests that amount to just calling
abidw --abidiff.  So, let's use that, now that we have it.

	* tests/test-read-dwarf.cc (handle_in_out_spec): Rather than
	calling abilint on the abixml and abidiff-ing the .so file against
	its .so.abi, call abidw --abidiff on the .so file and voila.  Ok,
	it does one extra save of abixml, but then that won't hurt.  And
	things are faster now than what they were anyway :-)

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