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.

Reviewed-by: Gary Lockyer <gary@catalyst.net.nz>
Signed-off-by: Philipp Gesang <philipp.gesang@intra2net.com>
22 jobs for phgsng-net-ads-json in 255 minutes and 2 seconds (queued for 1 second)
Status Name Job ID Coverage
  Build
passed build_ctdb #98258473
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failed build_samba_nt4 #98258487
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