Merge branch 'hl/test-helper-fd-close'

A few file descriptors left unclosed upon program completion in a
few test helper programs are now closed.

* hl/test-helper-fd-close:
  test-delta: close output descriptor after use
  test-delta: use strbufs to hold input files
  test-delta: handle errors with die()
  t/helper/test-truncate: close file descriptor after truncation