helper/structure: More cases for NormalizeJsonString tests
helper/structure: More cases for NormalizeJsonString tests

In terraform-providers/terraform-provider-aws#2935, we have been cleaning code
duplication by benefiting from the "NormalizeJsonString" present in the "structure" helper.

It appears that tests in the AWS provider are covering more use-cases,
which are added in this work.
hashicorp/terraform-plugin-sdk,hashicorp/terraform-plugin-sdk