tests/shell: prettify JSON in test output and add helper

tests/shell: prettify JSON in test output and add helper

- add helper script "json-pretty.sh" for prettify/format JSON.
  It uses either `jq` or a `python` fallback. In my tests, they
  produce the same output, but the output is not guaranteed to be
  stable. This is mainly for informational purpose.

- add a "json-diff-pretty.sh" which prettifies two JSON inputs and
  shows a diff of them.

- in "test-wrapper.sh", after the check for a .json-nft dump fails, also
  call "json-diff-pretty.sh" and write the output to "ruleset-diff.json.pretty".
  This is beside "ruleset-diff.json", which contains the original diff.

Signed-off-by: Thomas Haller <thaller@redhat.com>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>