tests: shell: quote reference to array to iterate over empty string

tests: shell: quote reference to array to iterate over empty string

This patch restores coverage for non-interval set backend.

Use "${FLAGS[@]}" in loop, otherwise empty string is skipped in the
iteration. This snippet:

  FLAGS=("")
  available_flags FLAGS "single"

  for flags in "${FLAGS[@]}" ; do
          echo $flags
  done

... now shows the empty string:

  # bash test.sh

  interval

Fixes: ed927baa4fd8 ("tests: shell: skip pipapo set backend in transactions/30s-stress")
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>