helper/resource: Fixes for golangci-lint issues

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 {
	   ^
```

helper/resource/plugin.go

@@ -116,8 +116,9 @@ func runProviderCommand(ctx context.Context, t testing.T, f func() error, wd *pl
 
 	// This should not happen, but prevent panics just in case.
 	if factories == nil {
-		logging.HelperResourceError(ctx, "Provider factories are missing to run Terraform command. Please report this bug in the testing framework.")
-		t.Fatalf("Provider factories are missing to run Terraform command. Please report this bug in the testing framework.")
+		err := fmt.Errorf("Provider factories are missing to run Terraform command. Please report this bug in the testing framework.")
+		logging.HelperResourceError(ctx, err.Error())
+		return err
 	}
 
 	// Run the providers in the same process as the test runner using the

helper/resource/testing_new.go

@@ -193,7 +193,7 @@ func runNewTest(ctx context.Context, t testing.T, c TestCase, helper *plugintest
 		if step.ImportState {
 			logging.HelperResourceTrace(ctx, "TestStep is ImportState mode")
 
-			err := testStepNewImportState(ctx, t, c, helper, wd, step, appliedCfg, providers)
+			err := testStepNewImportState(ctx, t, helper, wd, step, appliedCfg, providers)
 			if step.ExpectError != nil {
 				logging.HelperResourceDebug(ctx, "Checking TestStep ExpectError")
 				if err == nil {

helper/resource/testing_new_import_state.go

@@ -14,7 +14,7 @@ import (
 	"github.com/hashicorp/terraform-plugin-sdk/v2/terraform"
 )
 
-func testStepNewImportState(ctx context.Context, t testing.T, c TestCase, helper *plugintest.Helper, wd *plugintest.WorkingDir, step TestStep, cfg string, providers *providerFactories) error {
+func testStepNewImportState(ctx context.Context, t testing.T, helper *plugintest.Helper, wd *plugintest.WorkingDir, step TestStep, cfg string, providers *providerFactories) error {
 	t.Helper()
 
 	spewConf := spew.NewDefaultConfig()