tests: tail: avoid failure on Solaris 11

tests: tail: avoid failure on Solaris 11

* tests/tail/tail-c.sh: On Solaris 11, tail -c 4096 /dev/urandom,
will induce an lseek(,-4096,SEEK_END) which returns -4096 without
setting errno, and a subsequent read() then gives EINVAL.
Since tailing the end of a psuedo device is an edge case,
we just verify that we don't spin reading the device forever.