tests: change SAMPLE_SIZE type to libc_off_t

tests: change SAMPLE_SIZE type to libc_off_t

As ftruncate libc function and our create_sample function that calls
ftruncate both take size argument of type libc_off_t, change the type
of SAMPLE_SIZE constant to libc_off_t.

* tests/fstat.c (SAMPLE_SIZE): Cast to libc_off_t.
* tests/lstat.c (SAMPLE_SIZE): Likewise.
* tests/oldfstat.c (SAMPLE_SIZE): Likewise.
* tests/oldlstat.c (SAMPLE_SIZE): Likewise.
* tests/oldstat.c (SAMPLE_SIZE): Likewise.
* tests/stat.c (SAMPLE_SIZE): Likewise.
* tests/xstatx.c (SAMPLE_SIZE): Likewise.