tests: Add test for qdict_flatten()

Add a test case for qdict_flatten() in tests/check-qdict.c. This test
case covers the flattening of subordinate QLists as well.

Signed-off-by: Max Reitz <mreitz@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>
