Avoid test failure on Linux when STDIN is /dev/null

Avoid test failure on Linux when STDIN is /dev/null

sysseek($fh, -1, 0) succeed on Linux when $fh is a handle to /dev/null.
In our build environment the perl test suite runs with STDIN redirected
from /dev/null and without a /dev/tty.  The hope is that the same seek
on a regular file will always fail.