feat(anta.tests): Added testcase for MLAG primary priority

[MaheshGSLAB]

Description

Added testcase for MLAG primary priority

Fixes #529
Success: The test will pass if the MLAG state is set as 'primary' and the priority matches the input.
Failure: The test will fail if the MLAG state is not 'primary' or the priority doesn't match the input.
Skipped: The test will be skipped if MLAG is 'disabled'.

Checklist:

• My code follows the style guidelines of this project
• I have performed a self-review of my code
• I have run pre-commit for code linting and typing (pre-commit run)
• I have made corresponding changes to the documentation
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes (tox -e testenv)

[carl-baillargeon]

LGTM!