Fix testQosSaiDwrr issue

[JibinBao]

Description of PR

Fix issue:#11156

1. When sending a burst packet, sometimes packets will lost on the server side, which will cause a test fail, so change the port speed to decrease the burst rate
2. When the previous test case just sends a large of packets only by one queue such as queue1, then Dwrr test might fail, because queue1 has got much chance to send packets before, so it will get less chance to send packets than expected. Therefore add one first run as a dry run, and it will not check Dwrr function. After the dry run, all tested queues can be scheduled so that all queues are at the same start.

Summary:
Fixes # (issue)#11156

Type of change

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

Back port request

• 201911
• 202012
• 202205
• 202305

Approach

What is the motivation for this PR?

Fix testQosSaiDwrr issue: #11156

How did you do it?

1. Add one parameter port_target_speed. when port_target_speed is set, change the port speed to the specified one.
2. Dry run before real test

How did you verify/test it?

Run testQosSaiDwrr

Any platform specific information?

Any

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

Documentation