tests: net: mqtt: Group existing MQTT 3.1.1 test suites

Currently existing tests suites verify MQTT 3.1.1 functionality,
therefore group them together under "mqtt/v3_1_1" directory, so
that the test base can be extended easily in the future with MQTT 5.0
tests without collision.

Signed-off-by: Robert Lubos <robert.lubos@nordicsemi.no>