test that 'remove' throws on error

Trying to remove a directory that definitely exists is a simple way of
definitely causing an error.
test removing a file called '0'
fix 'remove' throwing

|| was checking whether the filename was itself true, rather than whether
unlink succeeded.
'remove' doc fix

By default C<unlink> never throws, so avoid implying that it would throw
when trying to delete a non-existent file.