l2cap-tester: Add SCI mode tests for LE server connections

Add L2CAP LE server connection tests that exercise the peripheral
side of the Shorter Connection Interval feature. The emulated client
(central) is configured via bthost_le_conn_rate() to send an LE
Connection Rate Request once connected, using the HOGP v1.2
recommended values for each of the four HID SCI modes (Fast, Default,
Low Power, Full Range).