[E2E Test Visibility] Fix TestMetadata UUID reuse causing DynamoDB records to be overwritten

[hehe7318]

Description of changes

Previously the UUID is computed once at module load time. When a worker runs test A then test B, both get the same DynamoDB key, and B's put_item overwrites A's record.

Now it generates a new UUID per instance.

Checklist

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Are you sure the new change can guarantee consistent UUID with in a single test?

[hehe7318]

Answer Hanwen's question:
Regarding the setup, call, and teardown phases, the code creates the TestMetadata object only once during the setup phase, so there are no issues here. See the code at: https://github.com/aws/aws-parallelcluster/blob/develop/tests/integration-tests/conftest_utils.py#L184-L231