rewriting test to cover error scenario

Signed-off-by: Akanksha Gahalot <akankshagahlot0307@gmail.com>

internal/credential/store.go

@@ -19,8 +19,6 @@ import (
 	credentials "github.com/oras-project/oras-credentials-go"
 )
 
-var CreateNewStore = credentials.NewStore
-
 // NewStore generates a store based on the passed-in config file paths.
 func NewStore(configPaths ...string) (credentials.Store, error) {
 	opts := credentials.StoreOptions{AllowPlaintextPut: true}
@@ -31,7 +29,7 @@ func NewStore(configPaths ...string) (credentials.Store, error) {
 
 	var stores []credentials.Store
 	for _, config := range configPaths {
-		store, err := CreateNewStore(config, opts)
+		store, err := credentials.NewStore(config, opts)
 		if err != nil {
 			return nil, err
 		}

internal/credential/store_test.go

@@ -14,27 +14,37 @@ limitations under the License.
 package credential
 
 import (
-	"fmt"
+	"os"
 	"reflect"
 	"testing"
-
-	credentials "github.com/oras-project/oras-credentials-go"
 )
 
 func TestNewStoreError(t *testing.T) {
-	// save current function
-	oldFunction := CreateNewStore
-	// restoring changes
-	defer func() { CreateNewStore = oldFunction }()
-	CreateNewStore = func(configPath string, opts credentials.StoreOptions) (credentials.Store, error) {
-		return nil, fmt.Errorf("New Error")
+	filename := "testfile.txt"
+	_, err := os.Create(filename)
+	if err != nil {
+		t.Errorf("error while creating test file %v", err)
+	}
+	defer func() {
+		err := os.Remove(filename)
+		if err != nil {
+			t.Errorf("error while removing test file %v", err)
+		}
+	}()
+
+	err = os.Chmod(filename, 000)
+	if err != nil {
+		t.Errorf("error: cannot change file permissions: %v", err)
 	}
-	var config string = "testconfig"
+
+	var config string = filename
 	credStore, err := NewStore(config)
+
 	if !reflect.DeepEqual(credStore, nil) {
 		t.Errorf("Expected NewStore to return nil but actually returned %v ", credStore)
 	}
 	if reflect.DeepEqual(err, nil) {
 		t.Error("Expected Error to be not nil but actually returned nil ")
 	}
+
 }