sudo testcli: Use hand-crafted JSON for output so that the test CLI is usable in tests
    
    The sudo testcli tool can be handy in tests, but currently its output is
    hard to process from a program. This patch makes the tool print an JSON
    output instead, which will make it more usable.
    
    Related:
    https://pagure.io/SSSD/sssd/issue/3596
    
    Reviewed-by: Pavel Březina <pbrezina@redhat.com>