Bluetooth: llcp: test: fix bsim test for eatt

The Babblesim test for eatt for reconfiguration of MTU is not
foolproof. Central actually generates two mtu update events.
This is fixed by only setting the reconfigured flag when the MTU actually
changes to the new values

In the Babblesim test for eatt for collision the peripheral
needs to sync with central to ensure collisions occur.

Signed-off-by: Andries Kruithof <andries.kruithof@nordicsemi.no>