tests: Bluetooth: Add ATT sequential procedures test

The purpose of this test is to prove that the zephyr host can handle the
behavior described in the Bluetooth Core Specification Vol.3 Part.F
3.3.2 "Sequential protocol".

The host should be able to handle all of those in parallel: one
indication, one write request, multiple commands and multiple
notifications.

The "tester" part had to be written with a "tiny host" implementation
instead of the Zephyr host, as the ZH conflates GATT client and server
and doesn't allow a device to enqueue an ATT request + an ATT indication
on the same bearer.

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