[dualtor-io] Support using fix source IP for upstream packets

[lolyu]

Description of PR

Summary:
Fixes # (issue)

Type of change

• Bug fix
• Testbed and Framework(new/improvement)
• Test case(new/improvement)

Back port request

• 202012
• 202205
• 202305
• 202311
• 202405

Approach

What is the motivation for this PR?

Let's make the packets for each server belongs to same TCP flow, SONiC will use the same route to forward it.
So any route change will forward/drop all the packets from the same server.

How did you do it?

Use fixed IP to generate packets for a single server.

How did you verify/test it?

dualtor_io/test_tor_bgp_failure.py::test_active_tor_kill_bgpd_upstream[active-standby] PASSED                                                                                                                                                                          [100%]^

Any platform specific information?

Supported testbed topology if it's a new test case?

Documentation