check-qdict: Test cases for new functions

This adds test cases for the following new QDict functions:

* qdict_array_entries()
* qdict_set_default_str()
* qdict_copy_default()

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