Bootstrap a unit-test suite (tests/unit) for the dtc plugins

[dacasti2mx]

Summary

The collection has no unit-test harness. CI currently runs only build,
sanity, and lint (.github/workflows/main.yml). Some plugin-level behaviors
(for example, the config-save error policy) can only be tested
deterministically with unit tests and mocks — the live-NDFC integration
tests cannot reproduce them reliably.

Scope

• Add a tests/unit/ layout following the ansible-test units convention.
• Add the CI job, Python dependencies, and Ansible mocking needed to run it.
• Document how to run the suite locally.

First case to cover: #813 (non-fatal config-save policy)

• 500 -> fabric_deploy_manager returns failed=True with the response intact
• 500 -> pipeline_base._config_save returns non-fatal
• 400 -> fatal
• missing/malformed msg -> fatal
• 200 -> ok

Context: the fix for #813 was verified locally with a temporary mock
harness. This issue tracks turning that into a permanent, CI-run test.

[dacasti2mx]

Heads-up for the first test case: the #813 fix now gates the non-fatal 500 on the data model (only tolerated when poap.preprovision is set). So the config-save matrix needs one extra row:

• 500 + pre-provisioned switches → non-fatal
• 500 + no pre-provisioned switches → fatal ← new

(400 / missing-malformed / 200 unchanged.)

[dacasti2mx]

Unit test successfully created in fork, once #819 is merged into develop, fix PR will be available in netascode/ansible-dc-vxlan repo.