memory_compare script is broken in 20.04

[Rod Smith]

Beginning with 20.04 pre-release versions, the memory_compare script (called by the memory/info job definition) is broken; it returns a failure and the following Python call traceback:

Traceback (most recent call last):
  File "./plainbox-provider-checkbox/bin/memory_compare", line 138, in <module>
    sys.exit(main())
  File "./plainbox-provider-checkbox/bin/memory_compare", line 101, in main
    installed_memory = HumanReadableBytes(get_installed_memory_size())
  File "./plainbox-provider-checkbox/bin/memory_compare", line 62, in get_installed_memory_size
    lshw.run(result)
  File "/usr/lib/python3/dist-packages/checkbox_support/parsers/lshwjson.py", line 47, in run
    self._parse_lshw(lshw, result)
  File "/usr/lib/python3/dist-packages/checkbox_support/parsers/lshwjson.py", line 34, in _parse_lshw
    if 'children' in lshw.keys():
AttributeError: 'list' object has no attribute 'keys'

This may be a regression of bug #1843063.

[Maciej Kisielewski (kissiel)]

An array is still a valid JSON output, so I'm fixing checkbox-support to make it work.

MR proposed here:
https://code.launchpad.net/~kissiel/checkbox-support/+git/checkbox-support/+merge/379856