tests/shell: have .json-nft dumps prettified to wrap lines

tests/shell: have .json-nft dumps prettified to wrap lines

Previously, the .json-nft file in git contains the output of `nft -j
list ruleset`. This is one long line and makes diffs harder to review.

Instead, have the prettified .json-nft file committed to git.

- the diff now operates on the prettified version. That means, it
  compares essentially

     - `nft -j list ruleset | json-sanitize-ruleset.sh | json-pretty.sh`
     - `cat "$TEST.json-nft" | json-pretty.sh`

  The script "json-diff-pretty.sh" is no longer used. It is kept
  however, because it might be a useful for manual comparing files.

  Note that "json-sanitize-ruleset.sh" and "json-pretty.sh" are still
  two separate scripts and called at different times. They also do
  something different. The former mangles the JSON to account for changes
  that are not stable (in the JSON data itself), while the latter only
  pretty prints it.

- when generating a new .json-nft dump file, the file will be updated to
  use the new, prettified format, unless the file is in the old format
  and needs no update. This means, with DUMPGEN=y, old style is preserved
  unless an update becomes necessary.

This requires "json-pretty.sh" having stable output, as those files are
committed to git. This is probably fine.

Signed-off-by: Thomas Haller <thaller@redhat.com>
Signed-off-by: Phil Sutter <phil@nwl.cc>