Add a fixture to disable thermal policy

[Junchao-Mellanox]

Description of PR

Summary:
Add a fixture to disable thermal policy

Type of change

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

Approach

What is the motivation for this PR?

In some test cases, thermal policy would override the mock value and cause test case failure sporadicly. So need a way to disable thermal policy during that test. The idea is to make thermalctld load an invalid thermal policy file. This PR provides a handy fixture to achieve this function.

How did you do it?

1. add a fixture to disable thermal policy
2. use this fixture in test_show_platform_fanstatus_mocked and test_device_checker

How did you verify/test it?

Manually run test_platform_info and test_snmp_phy_entity

Any platform specific information?

N/A

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

N/A

Documentation

[Junchao-Mellanox]

/AzurePipelines run

[azure-pipelines]

Commenter does not have sufficient privileges for PR 2953 in repo Azure/sonic-mgmt

[jleveque]

@Junchao-Mellanox: I'll run the test again.

[Junchao-Mellanox]

thanks!

[liat-grozovik]

/AzurePipelines run

[azure-pipelines]

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