tests: bluetooth: classic: Add test suite l2cap.

IUT works as a l2cap server with basic mode. The peer device,
l2cap client 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-case8 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 case9-case14 revolves around the basic
 parameters of L2CAPserver configuration, data transfer.
Case 9:   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 10: Test l2cap connection with min MTU(0x30),set server mtu = 48,
client mtu = 0xffff, l2cap connect successfully,
client mtu = 0xffff, server mtu = 48.
Case 11: Test l2cap connection with min MTU(0x30),set server mtu < 48,
client mtu = 0xffff, l2cap connect fail.
Case 12: Test l2cap connection with min MTU(0x30),set server mtu = 48,
client mtu<48,l2cap connect successfully,
client mtu=48,server mtu=48.
Case 13: Test l2cap connection with invaild PSM.
Case 14: Test l2cap multi_channel connection and data tranfer.
Case 15: Stress Test. Repeat l2cap connect, disconnect operation.
Case 16: Stress Test. Repeat data transfer in a single connection.
In Case 15 and 16, if enlarging STRESS_TEST_MAX_COUNT and test fail,
you can enlarge timeout in  testcase.yml.

test_l2cap_server.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>