Unittest of methods in main.py

[hlijuen]

Issue #, if available:
https://taskei.amazon.dev/tasks/V1358739306

Description of changes:
The file, test_cfn.py, containing test cases for various methods in main.py is added.
This file tests the following methods specified in main.py.

    get_policy_check_type()
    get_required_inputs(policy_check)
    get_optional_inputs(policy_check)
    get_sub_command(inputFields, areRequiredFields)
    get_treat_findings_as_non_blocking_flag(policy_check)
    build_command(policy_check, required_inputs=required_inputs, optional_inputs=optional_inputs
    execute_command(command)
    set_output(val)

Each test case sends in various input parameters to verify the correctness of the output from the tested methods.

The file, .github/workflows/unittest.yaml, is also added to run unit tests on pull _equest or push.