Add test-abidiff-exit/ld-2.28-21{0,1}.so to source distribution

Add test-abidiff-exit/ld-2.28-21{0,1}.so to source distribution

I forgot to update tests/data/Makefile.am when I added the test input
files test-abidiff-exit/ld-2.28-21{0,1}.so and
test-abidiff-exit/test-ld-2.28-210.so--ld-2.28-211.so.txt. Fixed thus.

Also, I am adding a new file that describes how to get the source code
of those binaries.

	* tests/data/Makefile.am: Add the new files to source
	distribution.  Also, add the new
	test-abidiff-exit/ld-2.28-21x.so.sources.txt.
	* tests/data/test-abidiff-exit/ld-2.28-21x.so.sources.txt: New
	file describing where the sources are.

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