[FEATURE] Improve testing for handling of manually deleted resources

[lechnerc77]

Improve the test scenarios for all resources READ operations for the handling of manually removed resources.

Scenario is:

• A resource is created via Terraform configuration (state is captured)
• The resource is deleted manually in the SAP BTP cockpit or via CLI (Terraform state still holds the information)
• A consequent READ should trigger a re-create of the entity as described in the best practices (see <>)

Testing this scenario is currently not possible via the testing framework (see hashicorp/terraform-plugin-testing#118 (comment)). This gap will be closed in a future release. After that the corresponding test cases must be added to our testing setup .