test: use config set_val_or_die instead of set_val

test: use config set_val_or_die instead of set_val

...in places that the return code was not being checked.

This fixes cases where an error in the config schema or the
value being passed in would cause weird failures beacuse the
set_val had not taken effect.

Signed-off-by: John Spray <john.spray@redhat.com>