tests: net: mqtt: Make publisher/subscriber test suites self contained

The MQTT publisher/subscriber/pubsub tests suites are currently
depending on an external MQTT broker being available. In result, the
test suites cannot be executed in twister and need to be run manually,
which makes them kind of pointless from the CI perspective.

This commit reworks the tests, so that they no longer rely on an
external broker being available, but rather implement minimalistic
broker functionality for test purposes only. This will make the tests
self contained, and thus executable in the CI.

Since it makes not point to duplicate the effort between the test
suites, the test suites have been merged into a single mqtt_client test
suite.

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