[platform/test_sfp]Operate SFP only once for split ports when resetting or setting LPM

[stephenxs]

Description of PR

Summary:
Fixes # (issue)
For split ports, only operate them once when testing reset and low power mode.

Type of change

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

Approach

How did you do it?

1. introduce a snippet of script which retrieves port mapping from DUT
2. get_port_map:

• for the first time being called, it copies the script to the DUT, executes it on DUT and then parses the result to construct the port mapping. It also saves the map for future use
• for the following time being called, it just returns the saved map.

3. when reset/set LPM for SFP modules, it uses the following logic to avoid repeating operating the physical SFP modules offering split ports:

    operated_phy_set = {}
    for each logical port
        phyport = port_mapping[logical]
        if phyport in operated_phy_set,
            skip
        else
            operate the SFP
            add phyport to operated_phy_set

How did you verify/test it?

run test_sfp

Any platform specific information?

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

Documentation

2100.split.extracted.2.txt
2700.split.extract.txt
3800.split.extract.txt
2010.split.extract.txt
2100.split.extract.txt