tests: bsim: bluetooth: mesh: Add short delay before suspending stack

The stack manages to suspend the advertiser before it finishes
transmitting the Outbound PDU Report message to confirm the transmission
of a Provisioning PDU. The test requires the server to become
unresponsive when the Provisioning PDU is sent to the unprovisioned
device to test timeout of the provisioning protocol.

Signed-off-by: Pavel Vasilyev <pavel.vasilyev@nordicsemi.no>