tests: Bluetooth: CAP: Test bt_cap_unicast_group_get_info

Add an additional step when doing CAP unicast tests where
we validate that the presentation delay set for each stream
match the presentation delay from bt_bap_unicast_group_get_info
via the BAP unicast group pointer retrieved via
bt_cap_unicast_group_get_info.

Signed-off-by: Emil Gydesen <emil.gydesen@nordicsemi.no>