[Nvidia] Update lpmode tests

[JibinBao]

Description of PR

Update lpmode tests

1. Update lpmode tests due to the design change for Nvidia devices: [Mellanox] Use sysfs for sfp reset/LPM/presence sonic-net/sonic-buildimage#14130
   a) For Nvidia devices, before setting lpmode we need to shutdown admin ports
   b) Add one step to check when setting lpmode to enabled with admin port up, lpmode is still off and port is still up.
2. Fix issue of test_check_sfputil_low_power_mode. Previously after setting the lpmode, test checks the value is on or off, which is not correct. we should check if the value of lpmode is the set one.
3. Optimize test of test_check_sfputil_low_power_mode, reduce time for looking up if port supports lpmode. Previously it was needed to look up two times, currently only one time.
4. Fix test issue for functions of links_status_down and links_status_up by stripping the space for every output

Summary:
Fixes # (issue)

Type of change

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

Back port request

• 201911
• 202012
• 202205

Approach

What is the motivation for this PR?

Update lpmode tests due to desgin change for Nvidia platform

How did you do it?

Update regarding tests according to the design

How did you verify/test it?

Run tests regarding lpmode on the image including new desgin

Any platform specific information?

Any

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

Any

Documentation

[JibinBao]

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