test: add functional tests for autoping (XEP-0199)

- autoping_set_interval: verify /autoping set command
- autoping_set_zero_disables: verify disabling autoping
- autoping_timeout_set: verify /autoping timeout command
- autoping_timeout_zero_disables: verify disabling timeout
- autoping_sends_ping_after_interval: verify automatic ping IQ
- autoping_server_not_supporting_ping: verify error handling

Fast tests (command parsing) in Group 1, slow tests (timer-based) in Group 3.

tests/functionaltests/test_autoping.h

@@ -0,0 +1,6 @@
+void autoping_set_interval(void** state);
+void autoping_set_zero_disables(void** state);
+void autoping_timeout_set(void** state);
+void autoping_timeout_zero_disables(void** state);
+void autoping_sends_ping_after_interval(void** state);
+void autoping_server_not_supporting_ping(void** state);