Add missing unit tests

[tcalmant]

The current unit tests cover around 84% of the framework, and will never be more than 95%, most of the untested lines being error handling in very specific/hardly reproducible cases.

However, there are still some services that are not tested at all, being:

• XMPP utility client and XMPP shell interface:
  • this is mainly due to the lack of an XMPP server in the test environment
  • it could be possible to install prosody or ejabberd to implement tests
• MQTT service and MQTT EventAdmin bridge
  • an MQTT server is available during tests, as MQTT Remote Services are tested
  • if it not used widely, the MQTT service could be removed as it is not used internally and it seems better to use paho-mqtt directly

The coverage of some unit tests doesn't seem to be taken into account when using a secondary Python process (e.g the shell console). This issue should be investigated in the same time.

• [X] MQTT Service
• [x] MQTT EventAdmin
• [x] XMPP Client
• [x] XMPP Shell

[tcalmant]

All modules/bundles of Pelix/iPOPO are now tested, which guarantees they can be used in Python 2.7 and in Python 3.4 to 3.6.