tests: bsim: Bluetooth: Fix simulation length be greater than wait time

Fix some test simulation length be greater than wait time.
These tests completed within 12 seconds, 15 second wait
time is a good value with included margin and a 20 second
simulation time.

These tests failed on this PR CI.

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