refactor: cleanup CISShadowMonitorTest

Issue #, if available:

Description of changes:

Consolidate all test cases into a single, parameterized test, since the core testing flow is the same
Move shadow request/response behavior from test cases themselves into FakeIotShadowClient

Why is this change necessary:

It will be easier to add upcoming test functionality in future PRs:
- simulating shadow retrieval failures (get request succeeds, but receiving doesn't happen)
Makes CISShadowMonitorTest (subjectively) easier to comprehend and maintain

How was this change tested:

Any additional information or context required to review the change:

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aws-greengrass / aws-greengrass-client-device-auth