tests/helper/schema: Add unit testing covering existing behavior of ConflictsWith and list index/map key syntax
tests/helper/schema: Add unit testing covering existing behavior of ConflictsWith and list index/map key syntax

Reference: https://github.com/hashicorp/terraform-plugin-sdk/issues/71
Reference: https://github.com/bflad/tfproviderlint/issues/104

The `TestSchemaMap_InternalValidate` tests also include TODOs for potential schema validation improvements.

hashicorp/terraform-plugin-sdk,hashicorp/terraform-plugin-sdk