tests: close unused peer socket in the tests using socketpair

tests: close unused peer socket in the tests using socketpair

Otherwise, the subsequent call to wait will block, when the child
process exits abnormally.  Checked with the following script:

  for i in $(git grep -l socketpair); do
          if ! grep -q 'close.*\[' $i; then echo $i; fi
  done

Signed-off-by: Daiki Ueno <ueno@gnu.org>