tests: rewrite utimensat.test without relying on libc utimensat wrapper

tests: rewrite utimensat.test without relying on libc utimensat wrapper

The new test also does more rigorous testing.

* tests/utimensat.c: Stop including <assert.h> and <errno.h>, include
<unistd.h> and <asm/unistd.h>, check __NR_utimensat instead
of HAVE_UTIMENSAT.
(errstr): New variable.
(k_utimensat): New function, a thin wrapper around
syscall(__NR_utimensat).
(main): Use them instead of utimensat.
* tests/gen_tests.in (utimensat): New entry.
* tests/utimensat.test: Remove.
* tests/Makefile.am (DECODER_TESTS): Remove it.