tests: Bluetooth: add l2cap MPS test

This test verifies the l2cap MPS workaround: when the other
side (non-zephyr host) sends PDUs that are < MPS.

The DUT is a host, while the Tester is a thin layer over the controller,
allowing fine control over l2cap packet contents.

Note that this test fails as-is. It is fixed by the next commits in the PR.

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