tests: bluetooth: classic: Add test suite l2cap client.

IUT works as a l2cap client with basic mode. The peer device,
l2cap server with basic mode, is a PC with running bumble on it.
This test only performs the function of L2CAP basic mode.
Support multiple l2cap enerties in new shell l2cap_br,
which may support more test function.

In the test suite , there are two groups in test cases.
Group 1 Including case1-case24 focuses on connection and
disconnection around l2cap. The impact of active and passive
acl connectivity, disconnectivity and authentication as well as
disconnection from ACL without l2cap disconnect is tested.

Group2 Including case25-case31 revolves around the basic
 parameters of L2CAPserver configuration, data transfer.
Case 25: Test l2cap connection with max MTU(0xffff).But the max mtu
which the stack supports is (CONFIG_BT_BUF_ACL_RX_SIZE - 4U = 196).
Case 26:  Test l2cap multi_channel connection and data tranfer.
Case 27: Test l2cap connection with invaild PSM.
Case 28: Test l2cap re_connect after disconnected.
Case 29: Test l2cap connection with min MTU(0x30).set server mtu = 48,
client mtu=48. l2cap connect successfully,
both server and client mtu should be 48.
Case 30: Test l2cap connection with min MTU(0x30). set server mtu = 48,
client mtu=0x27. l2cap connect fail.
Case 31: Test l2cap connection with min MTU(0x30).set server mtu = 30,
client mtu=48.l2cap connect successfully,
both server and client mtu should be 48.

test_l2cap_client.py is a file containing real test cases.
test_l2cap_common.py is a common file. It encapsulates some
test function functions that are commonly used for test cases.

Signed-off-by: Cheng Chang <cheng.chang@nxp.com>