tests: Bluetooth: bsim: Increase to 250 simultaneous connections.

Increase to 250 simultaneous connections in the BabbleSim
test. Update connection interval to accommodate 250
non-overlapping connections and a scan window. And make the
use of is_disconnecting flag consistent between central and
peripheral sample.

Signed-off-by: Vinayak Kariappa Chettimada <vich@nordicsemi.no>