Bluetooth: Host: Test GATT read buffer size

This test ensures the data from a charachteristic read callback is not
truncated anywhere. This test illuminates an existing failure, so it's
failing in this commit. The fix is in the same PR.

The test comes with an experimental "library" with easy-to-use
wrappers/utilities for Bluetooth API. The library starts its life in the
directory of the test, but it will be moved outwards when more tests
start using it. It is designed to not interfere with other users of the
Bluetooth API. The library does work just as well on real hardware,
outside of BabbleSim, so it can be used for target tests and
proof-of-concept applications.

Signed-off-by: Aleksander Wasaznik <aleksander.wasaznik@nordicsemi.no>