TESTS: test the curl wrapper with a command-line tool
    
    In order to test the curl integration code, this patch adds a
    command-line tool and tests that it's possible to drive a conversation
    with the secrets responder using the tool.
    
    Reviewed-by: Pavel Březina <pbrezina@redhat.com>
    Reviewed-by: Lukáš Slebodník <lslebodn@redhat.com>