helper/resource: Fix duplicated function testConfig
helper/resource: Fix duplicated function testConfig

An earlier change introduced a new function testConfig to the main code
for this package, which conflicted with a function of the same name in
the test code.

Here we rename the function from the test code, allowing for the more
generally-named testConfig to be the one in the main code.

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