Split up serverless integration tests into dedicated package

[pkoutsovasilis]

As part of improving the organization and maintainability of our integration tests, we should extract the serverless-specific tests from the testing/integration package into their own package: testing/integration/serverless.

This will help clarify test ownership, simplify setup logic, and make it easier to expand or run serverless tests independently. This change is in line with the broader goal of modularising our integration test suite.