tests/resource/aws_transfer_user: Update user_name validation acceptance test name for consistency
tests/resource/aws_transfer_user: Update user_name validation acceptance test name for consistency

Reference: https://github.com/terraform-providers/terraform-provider-aws/pull/8304#pullrequestreview-230116962

terraform-providers/terraform-provider-aws,Ninir/terraform-provider-aws,terraform-providers/terraform-provider-aws,kjmkznr/terraform-provider-aws,kjmkznr/terraform-provider-aws,Ninir/terraform-provider-aws,kjmkznr/terraform-provider-aws,kjmkznr/terraform-provider-aws,terraform-providers/terraform-provider-aws,Ninir/terraform-provider-aws,terraform-providers/terraform-provider-aws,Ninir/terraform-provider-aws