Improve test DEB/RPM package speed

Test RPM/DEB package task is slow in batch CI, delaying our release process. We've used an 8-core agent, but the task still takes over an hour.

Using more cores won't significantly improve performance. Instead, we should focus on enhancing the test module.

- Some modules are run in serial, such as network.
- Some tests run much slower than others, slowing down the entire testing process. For example: `tests/latest/test_keyvault_commands.py::KeyVaultMgmtScenarioTest::test_keyvault_list_deleted` takes 6 min.

Related PR:
- https://github.com/Azure/azure-cli/pull/30238
- https://github.com/Azure/azure-cli/pull/30922