Reference: #253
Reference: #628
Reference: #779

Provider developers can now select whether to configure providers for acceptance testing at the `TestCase` or `TestStep` level. Only one level may be used in this current implementation, however it may be possible to allow merged `TestCase` and `TestStep` configuration with additional validation logic to ensure a single provider is not specified multiple times across the merge result of all fields.

This change also introduces some upfront `TestCase` and `TestStep` configuration validation when calling any of the `Test` functions, failing the test early if a problem is detected. There are other validations that are possible, however these are considered out of scope.

Previously:

```
func testStepNewImportState(ctx context.Context, t testing.T, c TestCase, helper *plugintest.Helper, wd *plugintest.WorkingDir, step TestStep, cfg string, providers *providerFactories) error {
                                                              ^
helper/resource/plugin.go:163:47: SA5011: possible nil pointer dereference (staticcheck)
	for providerName, factory := range factories.legacy {
	                                             ^
helper/resource/plugin.go:118:5: SA5011(related information): this check suggests that the pointer can be nil (staticcheck)
	if factories == nil {
	   ^
```

…et and before TestStep providers are changed

Previously:

```
--- FAIL: TestTest_TestStep_ExternalProviders_To_ProviderFactories_StateUpgraders (2.50s)
    /Users/bflad/src/github.com/hashicorp/terraform-plugin-sdk/helper/resource/teststep_providers_test.go:275: Step 2/2 error: exit status 1
        Failed to marshal state to json: schema version 0 for null_resource.test in state does not match version 1 from the provider
    /Users/bflad/src/github.com/hashicorp/terraform-plugin-sdk/helper/resource/testing_new.go:73: Error retrieving state, there may be dangling resources: exit status 1
        Failed to marshal state to json: schema version 0 for null_resource.test in state does not match version 1 from the provider
```