helper/resource: Create a separate provider instance each call
helper/resource: Create a separate provider instance each call

Previously we were running the factory function only once when
constructing the provider resolver, which means that all contexts created
from that resolver share the same provider instance.

Instead now we will call the given factory function once for each
instantiation, ensuring that each caller ends up with a separate object
as would be the case in real-world use.

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