tests/bsim: run_parallel: Do not set exe permissions if missing

This script tried to be too nice with users, and if they forgot
to set the scripts as executable, set it for them.
But this leads to too many testcases commited to main without
executable permissions.
Which then leads to other developers needing to set those
locally and end with dirty workspaces.

Instead let's not have run_parallel set the executable bit,
and have these tests fail in CI, so developers realize and fix it.

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