[thermalctld] Refactor to allow for greater unit test coverage; Add more unit tests

[jleveque]

Description

• Refactor thermalctld to reduce the amount of code in infinite loops, thus allowing us better unit test coverage

• Refactor mock_platform.py such that it inherits from sonic-platform-common in order to ensure it is aligned with the current API definitions (this introduces a test-time dependency on the sonic-platform-common package)

• Eliminate the need to check for a THERMALCTLD_UNIT_TESTING environment variable in daemon code

• Increase pytest verbosity to prevent truncation of error messages

• Miscellaneous cleanup:

  • Fixes to grammar
  • Remove unnecessary punctuation from log messages

• Increase overall unit test unit test coverage from 73% to 93%

Previous unit test coverage:

----------- coverage: platform linux, python 3.7.3-final-0 -----------
Name                  Stmts   Miss  Cover
-----------------------------------------
scripts/thermalctld     424    113    73%

Unit test coverage with this patch:

----------- coverage: platform linux, python 3.7.3-final-0 -----------
Name                  Stmts   Miss  Cover
-----------------------------------------
scripts/thermalctld     434     29    93%

Motivation and Context

As part of ongoing efforts to increase unit test coverage >= 90% across the SONiC codebase in order to prevent regressions

How Has This Been Tested?

The refactored thermalctld has been tested on a physical DUT and the unit test results can be examined via the Azure Pipelines check builds in this PR.

[Junchao-Mellanox]

How about we add a helper function like below:

def get_exception_message(msg):
    if sys.version_info.major == 3:
        return 'Exception(\'{},\')'.format(msg)
    else:
        return 'Exception(\'{}\')'.format(msg)

And we don't have to change everywhere when removing python2 support.

[jleveque]

Thank you for the suggestion. However, I believe this unit test code will not change much, if at all before we remove Python 2 support. And with things the way they are, all we have to do is delete lines. If we make the change you suggested, there will actually be a bit more work needed when removing Python 2 support. Therefore, I prefer we just leave things as-is for the time being.

[vdahiya12]

@jleveque this dependency is causing sonic-net/sonic-buildimage#7143 sonic-buildimage to fail.
Do we need to remove it or add something ?

[jleveque]

PR to add dependency in sonic-buildimage here