bsim tests: Fix incorrect libC free call from bstests

The bstests code builds and executes with the embedded
code, but had a call into "free()" which, when building
the embedded code with an embedded C library lands
in the embedded libC free, while it should
always call into the host libC free.

Fix it by calling thru the appropriate trampoline.

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