tests bsim/net: Add simple echo test over 802.15.4

Add a simple selfchecking test for the 15.4 stack,
based on the echo client / server sample apps.
It relies on the bs_tests hooks to register a test timer
callback, which after a deadline will check how many
packets the echo client has got back as expected, and if
over a threshold considers the test passed.

Signed-off-by: Alberto Escolar Piedras <alberto.escolar.piedras@nordicsemi.no>