[YANG] Add an infrastructure to write pytest tests for the YANG model.

[oleksandrivantsiv]

Why I did it

The existing JSON tests are limited and require a huge amount of copy-paste to provide good coverage. The Pytest provides an alternative way to implement the tests. The tests and more readable and human-friendly. They also give more flexibility in the testing of corner cases.

As an example, the CRM, DASH CRM, and SmartSwitch tests are converted to Pytest. The new tests are more compact and provide better coverage.

Work item tracking

• Microsoft ADO (number only):

How I did it

Add an infrastructure that allows to write model tests in Python.
Conver CRM, DASH CRM, and SmartSwitch tests to Pytest.

How to verify it

Build the YANG model package.

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

• 201811
• 201911
• 202006
• 202012
• 202106
• 202111
• 202205
• 202211
• 202305

Tested branch (Please provide the tested image version)

Description for the changelog

Link to config_db schema for YANG module changes

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