tests: Bluetooth: add sleep in notify cb testcase

Validates that nothing nefarious happens if the application decides to
wait an arbitrary amount of time in notification callbacks.

Also validates that a disconnection during said waiting works as
expected.

E.g. in #48505, the user was trying to send data synchronously over UART
_inside_ the notification callback. Then the device was disconnected
whilst still waiting.

This kind of pattern is not recommended, but the stack should still
function correctly, stalling the "BT RX" thread shouldn't result in
state corruption or resource leakage.

Signed-off-by: Jonathan Rico <jonathan.rico@nordicsemi.no>