Test_run_execution class operator variable are lazy loaded from DB

[raju-apple]

From chip-certification-tool-backend created by sholagi: CHIP-Specifications/chip-certification-tool-backend#296

In #289, we introduced a workaround to ensure operator was explicitly loaded (wasn't before due to lazy loading).

This should be refactored, to use from_orm in pydantic directly on the new model, once they allow us to pass additional values for the new model eg. test_run_stats. This will let the schema handle the loading of the operator. see: https://github.com/samuelcolvin/pydantic/pull/3375

Related PR: https://github.com/CHIP-Specifications/chip-certification-tool-backend/pull/289 Related Issue: https://github.com/CHIP-Specifications/chip-certification-tool-backend/issues/276