Bluetooth: host: add l2cap stress-test

This test reproduces more-or-less #34600.

It has a central that connects to multiple peripherals, opens one l2cap CoC
channel per connection, and transmits a few SDUs largely exceeding the MPS
of the channel.

In this commit, the test doesn't pass, but when it passes (after the
subsequent commits), error and warning messages are expected from the
stack, as this is not the happy path.

We can later debate on whether these particular error messages should be
downgraded to debug.

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