Add a modem test that uses fake devices to do full send/recv tests

Adds tests on ipv6/ipv4 to test receiving messages from fake
devices running on john.  You will need ipv6 for half of the modem
tests to pass.

Pair-programmed with @jaredmorrow
Merge pull request #60 from helium/cunit-echo-test

Add a modem test that uses fake devices to do full send/recv tests

Reviewed-by: jaredmorrow