tests/bsim bt host/att: Test scripts cleanup

Use the common scripts so we have a consitent way of
running the tests, report failures in the same way,
can stop them with ctrl+C, properly wait for
all processes to exit..
Let's also place the test scripts in the test_scripts/
folder as it is convention.

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

(f) ests/bsim bt att:  Use common s