tests: rewrite sched_xetattr.test from match_grep to match_diff

tests: rewrite sched_xetattr.test from match_grep to match_diff

Unlike the previous edition of the test that was based on match_grep,
the new one is match_diff based and does more rigorous testing.

* tests/adjtimex.c: Include "xlat.h" and "xlat/schedulers.h",
(sys_sched_getattr, sys_sched_setattr): New functions.
(main): Use them.  Update expected output.
* tests/adjtimex.test: Use run_strace_match_diff.