boards: bsim: fix bug with single settings file for few bsim devices

Reapply the part of 4b5cd923
that added new functionality/fixed the actual issues described
in the original commit message withtout the further cleanup.

There was also some cross-commit content from
2b91ebe1
as part of the cleanup in test_friendship.c which is also included.
+ added cleanup also to test_provision.c on request from Aleksandr.

Original commit msg:
-----------
Multiple bsim devices cannot store individual settings
with the existing settings backend for bsim.

Signed-off-by: Aleksandr Khromykh <aleksandr.khromykh@nordicsemi.no>
-----------

Signed-off-by: Alberto Escolar Piedras <alpi@oticon.com>