tests/shell: check and generate JSON dump files

tests/shell: check and generate JSON dump files

The rules after a successful test are good opportunity to test
`nft -j list ruleset` and `nft -j --check`. This quite possibly touches
code paths that are not hit by other tests yet.

The only downside is the increase of the test runtime (which seems
negligible, given the benefits of increasing test coverage).

Future commits will generate and commit those ".json-nft" dump files.

"DUMPGEN=y" will, like before, regenerate only the existing
"*.{nodump,nft,json-nft}" files (unless a test has none of the 3 files,
in which case they are all generated and the user is suggested to commit
the correct ones). Now also "DUMPGEN=all" is honored, that will generate
all 3 files, regardless of whether they already existed. That is useful
if you start out with a test that only has a .nft file, and then you
want to generate a .json-nft file too.

Signed-off-by: Thomas Haller <thaller@redhat.com>
Signed-off-by: Florian Westphal <fw@strlen.de>