test: Fix fd plumbing

test: Fix fd plumbing

In a background process, stdin defaults to /dev/null, so we need to do
a little dance.  <&0 looks like it ought to work according to the bash
manual and does indeed work in bash, but not in dash.  This
construction seems to work in both.

Tested with:
  date | sh -exc 'cat <&0 >t &'

SuS says:
  If job control is disabled (see set, -m), the standard input for an
  asynchronous list, before any explicit redirections are performed,
  shall be considered to be assigned to a file that has the same
  properties as /dev/null. This shall not happen if job control is
  enabled. In all cases, explicit redirection of standard input shall
  override this activity.

Signed-off-by: Ian Jackson <ijackson@chiark.greenend.org.uk>