add difftest.sh test

The purpose of this test script is to determine if create-diff-object
can properly recognize object file equivalence when passed the same file
for both the original and patched objects.  This verifies that
create-diff-object is correctly parsing, correlating, and comparing the
different elements of the object file.  In practice, a situation similar
to the test case occurs when a commonly included header file changes,
causing Make to rebuild many objects that have no functional change.

Signed-off-by: Seth Jennings <sjenning@redhat.com>