tests/bsim bt host/misc: Tests scripts cleanup

Ensure we call the common source script before using
the BOARD variable;
Use the common scripts to perform common functions
and do a cleanup.
Do not set variables to their defaults, set things
closer to were they are needed;
Do not perform unnecesary checks;
And in general avoid unnecessary complexity.

And return !=0 on error.

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