[dualtor][test gap] Add testcases to cover the faulty ycable (sonic-net#14680)

What is the motivation for this PR?
To fix test gap: sonic-net#12311

How did you do it?
To simulate a faulty ycabled, a templated y_cable_simulated.py is used to replace the original ycable driver.
With the faulty ycable driver, two testcases are added:

add test_switchover_probe_unknown to verify dualtor could toggle as expected even ycabled returns mux probe unknown.
add test_switchover_peer_link_down to verify dualtor could toggle as expected even ycabled returns peer link down.

How did you verify/test it?
dualtor/test_switchover_faulty_ycable.py::test_switchover_probe_unknown PASSED                                                                                                                                                                       [ 50%]
dualtor/test_switchover_faulty_ycable.py::test_switchover_peer_link_down PASSED                                            

Signed-off-by: Longxiang Lyu <lolv@microsoft.com>

Author: lolyu