[determine-reboot-cause] Skip invoking platform code for unit tests for hardware_reboot_cause

[vaibhavhd]

Fix unit tests in determine-reboot-cause to skip calling platform module libraries in unit test scope.

- Why I did it
MELLANOX build is failing for the recent PRs. The errors are due to platform library being invoked in a unit test for determine-reboot-cause script.

[2020-12-30T11:03:13.516Z] tests/determine-reboot-cause_test.py:102: 
[2020-12-30T11:03:13.516Z] _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ 
[2020-12-30T11:03:13.516Z] scripts/determine-reboot-cause:107: in find_hardware_reboot_cause
[2020-12-30T11:03:13.516Z]     platform  = sonic_platform.platform.Platform()
[2020-12-30T11:03:13.516Z] /usr/local/lib/python3.7/dist-packages/sonic_platform/platform.py:21: in __init__
[2020-12-30T11:03:13.516Z]     self._chassis.initialize_components()
[2020-12-30T11:03:13.516Z] /usr/local/lib/python3.7/dist-packages/sonic_platform/chassis.py:167: in initialize_components
[2020-12-30T11:03:13.516Z]     self._component_list.extend(ComponentCPLD.get_component_list())
[2020-12-30T11:03:13.516Z] /usr/local/lib/python3.7/dist-packages/sonic_platform/component.py:745: in get_component_list
[2020-12-30T11:03:13.516Z]     cpld_number = cls._read_generic_file(cls.CPLD_NUMBER_FILE, cls.CPLD_NUMBER_MAX_LENGTH)
[2020-12-30T11:03:13.516Z] _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ 

[2020-12-30T11:03:13.516Z]         try:
[2020-12-30T11:03:13.516Z]             with io.open(filename, 'r') as fileobj:
[2020-12-30T11:03:13.516Z]                 result = fileobj.read(len)
[2020-12-30T11:03:13.516Z]         except IOError as e:
[2020-12-30T11:03:13.516Z]             if not ignore_errors:
[2020-12-30T11:03:13.516Z] >               raise RuntimeError("Failed to read file {} due to {}".format(filename, repr(e)))
[2020-12-30T11:03:13.516Z] E               RuntimeError: Failed to read file /var/run/hw-management/config/cpld_num due to FileNotFoundError(2, 'No such file or directory')

- How I did it
Modified unit tests to use mock utility to skip calling sonic_platform libraries.

- How to verify it
Executed unit tests and Mellanox PR build expected to pass now.

- Which release branch to backport (provide reason below if selected)

• 201811
• 201911
• 202006

- Description for the changelog

- A picture of a cute animal (not mandatory but encouraged)

[lguohan]

for unit test failure, better to patch chassis.get_reboot_cause()

[qiluo-msft]

ImportError [](start = 11, length = 11)

I think you use ImportError to determine test environment. This is fragile.
You'd better manipulate libraries in the test code, and not change scripts under test.
There are samples in https://github.com/Azure/sonic-snmpagent

[vaibhavhd]

I agree that ImportError handling is not the best way to determine test env. In this change, based on the discussion offline, we have decided to use mock function to avoid hitting this code at all.