tests/blackbox: add test for net ads JSON output

Implement blackbox tests for

    $ net ads info --json
    $ net ads lookup --json

that validate

    a) JSON wellformedness (by feeding it into the JSON library
       that ships with Python), and
    b) equality of the set of keys printed to that of the
       non-JSON version.

Signed-off-by: Philipp Gesang <philipp.gesang@intra2net.com>
20 jobs for phgsng-net-ads-json in 251 minutes and 16 seconds (queued for 1 second)
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