tests/bt/bsim/mesh: Fix tests dependent on RSSI

The friendship tests are dependent on the attenuation
level as they take (much) longer with higher channel
attenuation.

As BabbleSim's "magic" modem model (default one used in these
tests) has been updated to provide a somehow accurate
RSSI measurement, one of these tests fails
tests/bluetooth/bsim/mesh/tests_scripts/friendship/msg_mesh_low_lat.sh
fails with the default channel attenuation of 60dBs.
Reduce the channel attenuation to 35dBs to approach the
previous RSSI values this tests were seeing on the DUTs with the
default modem.

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