feat: Unit test resource and data source schemas in Terraform Plugin Framework

[lantoli]

Description

Jira ticket: INTMDB-1306

Unit test resource and data source schemas in Terraform Plugin Framework, more info here.

Type of change:

• Bug fix (non-breaking change which fixes an issue). Please, add the "bug" label to the PR.
• New feature (non-breaking change which adds functionality). Please, add the "enhancement" label to the PR.
• Breaking change (fix or feature that would cause existing functionality to not work as expected). Please, add the "breaking change" label to the PR.
• This change requires a documentation update
• Documentation fix/enhancement

Required Checklist:

• I have signed the MongoDB CLA
• I have read the contribution guidelines
• I have added tests that prove my fix is effective or that my feature works per HashiCorp requirements
• I have added any necessary documentation (if appropriate)
• I have run make fmt and formatted my code
• If changes include deprecations or removals, I defined an isolated PR with a relevant title as it will be used in the auto-generated changelog.

Further comments

Example of schema problem detection:

--- FAIL: TestDataSourceSchemas (0.00s)
    --- FAIL: TestDataSourceSchemas/mongodbatlas_database_users (0.00s)
        fw_provider_test.go:70: Schema validation diagnostics: [{detail:When validating the resource or data source schema, an implementation issue was found. This is always an issue with the provider and should be reported to the provider developers.
            
            "count" is a reserved root attribute/block name. This is to prevent practitioners from needing special Terraform configuration syntax. summary:Reserved Root Attribute/Block Name}]
--- FAIL: TestResourceSchemas (0.00s)
    --- FAIL: TestResourceSchemas/mongodbatlas_database_user (0.00s)
        fw_provider_test.go:38: Schema validation diagnostics: [{detail:When validating the resource or data source schema, an implementation issue was found. This is always an issue with the provider and should be reported to the provider developers.
            
            "connection" is a reserved root attribute/block name. This is to prevent practitioners from needing special Terraform configuration syntax. summary:Reserved Root Attribute/Block Name}]
    --- FAIL: TestResourceSchemas/mongodbatlas_alert_configuration (0.00s)
        fw_provider_test.go:38: Schema validation diagnostics: [{detail:When validating the resource or data source schema, an implementation issue was found. This is always an issue with the provider and should be reported to the provider developers.
            
            "count" is a reserved root attribute/block name. This is to prevent practitioners from needing special Terraform configuration syntax. summary:Reserved Root Attribute/Block Name}]
FAIL

[github-actions]

Package	Line Rate	Health
github.com/mongodb/terraform-provider-mongodbatlas/mongodbatlas	3%	❌
github.com/mongodb/terraform-provider-mongodbatlas/mongodbatlas/framework/validator	68%	➖
github.com/mongodb/terraform-provider-mongodbatlas/mongodbatlas/testutils	46%	❌
github.com/mongodb/terraform-provider-mongodbatlas/mongodbatlas/util	17%	❌
Summary	3% (328 / 10541)	❌

[AgustinBettati]

looks good!