tests: Bluetooth: BR: Add test suite of SC Only Mode for SMP

IUT works as an SM Initiator/Responder. The peer device,
SM Responder/Initiator, is a PC running bumble on it.

Add shell commands to create and manage L2CAP server and channel with
different security level.

In the test suite, there are 10 test cases. IUT is configured
with Secure Connections Only Mode. The peer device is configured with
Secure Connections Mode or Legacy Mode. The test matrix is:
| Case ID | Secure Connection Option | Security Level | Expected Result |
|---------|--------------------------|----------------|-----------------|
| BR_SM_SC_INIT_005 | SC Only with SC | Level 4 | Pass |
| BR_SM_SC_INIT_006 | SC Only with SC | Level 1 | Fail |
| BR_SM_SC_INIT_007 | SC Only with SC | Level 2 | Fail |
| BR_SM_SC_INIT_008 | SC Only with SC | Level 3 | Fail |
| BR_SM_SC_INIT_009 | SC Only with Legacy | Level 3 | Fail |
| BR_SM_SC_RSP_005 | SC Only with SC | Level 1 | Fail |
| BR_SM_SC_RSP_006 | SC Only with SC | Level 2 | Fail |
| BR_SM_SC_RSP_007 | SC Only with SC | Level 3 | Fail |
| BR_SM_SC_RSP_008 | SC Only with SC | Level 4 | Pass |
| BR_SM_SC_RSP_009 | SC Only with Legacy | Level 3 | Fail |

Signed-off-by: Can Wang <can.wang@nxp.com>