tests/multi_bluetooth: Synchronise MTU exchange in BLE MTU tests.

With the recent update to ESP-IDF 5.4.2, there is a change in BLE event
behaviour which makes `tests/multi_bluetooth/ble_mtu.py` and
`tests/multi_bluetooth/ble_mtu_peripheral.py` now fail on ESP32 with IDF
5.4.2.

The change in behaviour is that MTU_EXCHANGE events can now occur before
CENTRAL_CONNECT/PERIPHERAL_CONNECT events.  That seems a bit strange,
because the MTU exchange occurs after the connection.  And looking at the
timing of the events there is exactly 100ms between them, ie MTU_EXCHANGE
fires and then exactly 100ms later CENTRAL_CONNECT/PERIPHERAL_CONNECT
fires.

It's unknown if this is a bug in (Espressif's) NimBLE, a subtle change in
scheduling with still valid behaviour, an intended change, a change allowed
under the BLE spec, or something else.

But in order to move forward with updating to IDF 5.4.2, the relevant tests
have been adjusted so they can pass.  The test just needs to wait a bit
between doing the connect and doing the MTU exchange, so the other side
sees the original/correct ordering of events.  This wait is done using the
multitest synchronisation primitives (broadcast and wait).

Signed-off-by: Damien George <damien@micropython.org>

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