net_test_bluetooth: RFCOMM test refactor

Properly implement RFCOMM tests into net_test_rfcomm using GUnit.
Contains all the RFCOMM tests and the base class that the tests use.
This test currently requires the DUT to be paired with an HFP capable
device.

Bug: 25793348
Change-Id: I683db7c5f2779373b2e343c7ad21268e4969d014