chore: updated new cross account id and entity guid

[RavitejaSurampudi]

Summary

This PR (updated on 27 Oct, 2023) addresses the following

• updates to the GUID used in the value of the linked_entity_guids argument in tests of dashboard resources to a GUID that is now active in the account we use to run integration tests (the values which currently exist are outdated, belong to an older account)
• discarding hardcoded account IDs in tests and at other points in the code, such as the example found in the AWS Govcloud Integrations resource and replacing such references with testAccountID and testSubAccountID
• refactoring in the tests of the newrelic_entity data source similar to the above (though the subaccount ID used here could not be eliminated as this is a second subaccount of testAccountID and is not comprised by GitHub secrets at the moment)