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)."

t/nqp/19-file-ops.t

@@ -76,7 +76,7 @@ my $ok := 1;
 try { nqp::seekfh($fh, -5, 0); $ok := 0; 1 }
 ok($ok, 'seekfh before start of file fails');
 $ok := 1;
-try { nqp::seekfh($fh, 0, 3); $ok := 0; 1 }
+try { nqp::seekfh($fh, 0, 5); $ok := 0; 1 }
 ok($ok, 'seekfh with invalid whence fails');
 nqp::closefh($fh);