AIX 7.1 supports diff -u, but its output does not match the expected
one.
Reported by Bruno Haible.
https://lists.gnu.org/r/bug-bison/2020-05/msg00049.html
* tests/atlocal.in (DIFF_U_WORKS): New.
* tests/local.at (AT_DIFF_U_CHECK): New.
* tests/existing.at (_AT_TEST_EXISTING_GRAMMAR): Use AT_DIFF_U_CHECK.