tests: Bluetooth: bsim: Handle consecutive connection in tests

If we want to test conn on legacy and ext adv, we need scanx to
handle connection more than once. Change local flag to global and
let it indicate whether we want scanx to initaite connection when
device is scanned.

Signed-off-by: Andrzej Kaczmarek <andrzej.kaczmarek@codecoup.pl>