tests: bsim: Bluetooth: Workaround overlapping ACL in BAP broadcast sink

Workaround by using `-start_offset` to ensure that the ACL
established does not overlap the sink being established.

The behavior between nrf52_bsim and nrf54l15bsim is
different where in the two device are starting to
advertising at the same time in nrf54l15bsim where as
there is an offset already in nrf52_bsim test.

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