tests: prepare for detailed testing of stat family syscalls

tests: prepare for detailed testing of stat family syscalls

Parametrize code from fstatat.c to support <asm/stat.h>
as an alternative to <sys/stat.h> and move it to a separate file.

* tests/statx.sh: New file, based on fstatat64.test.
* tests/fstatat64.test: Use it.
* tests/newfstatat.test: Likewise.
* tests/xstatx.c: New file, based on fstatat.c.
* tests/fstatat.c: Use it.
* tests/Makefile.am (EXTRA_DIST): Add statx.sh and xstatx.c.
* tests/fstatat64.c (FSTATAT_NAME): Change to TEST_SYSCALL_NAME.
* tests/newfstatat.c: Likewise.