tests/subsys/modem: Added unit tests for modem modules

The tests test each module independently using the mock pipe,
which implements the modem_pipe API, making it compatible
with the modem modules.

The modem_ppp module is slightly different from the others
since it also interacts with the network stack. To acheive
this, a mock implementation of the PPP L2 net iface has
been implemented in the test suite for the modem_ppp.

Signed-off-by: Bjarki Arge Andreasen <baa@trackunit.com>