adjust lseek test because of a change in lseek itself

'3' is not an invalid 'whence' anymore on some platforms, see:
http://man7.org/linux/man-pages/man2/lseek.2.html
"SEEK_DATA and SEEK_HOLE are nonstandard extensions also present in
Solaris, FreeBSD, and DragonFly BSD; they are proposed for inclusion
in the next POSIX revision (Issue 8)."