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

@@ -3,7 +3,9 @@ Copyright The ORAS Authors.
 Licensed under the Apache License, Version 2.0 (the "License");
 you may not use this file except in compliance with the License.
 You may obtain a copy of the License at
+
 http://www.apache.org/licenses/LICENSE-2.0
+
 Unless required by applicable law or agreed to in writing, software
 distributed under the License is distributed on an "AS IS" BASIS,
 WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
@@ -14,27 +16,33 @@ limitations under the License.
 package credential
 
 import (
-	"fmt"
+	"os"
+	"path"
 	"reflect"
+	"strings"
 	"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")
+	tmpDir := t.TempDir()
+	filename := path.Join(tmpDir, "testfile.txt")
+	_, err := os.Create(filename)
+	if err != nil {
+		t.Errorf("error: cannot create file : %v", err)
 	}
-	var config string = "testconfig"
-	credStore, err := NewStore(config)
-	if !reflect.DeepEqual(credStore, nil) {
+	err = os.Chmod(filename, 000)
+	if err != nil {
+		t.Errorf("error: cannot change file permissions: %v", err)
+	}
+	credStore, err := NewStore(filename)
+	if 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 ")
+	if err != nil {
+		ok := strings.Contains(err.Error(), "failed to open config file")
+		reflect.DeepEqual(ok, true)
+
+	} else {
+		t.Errorf("Expected err to be not nil")
 	}
 }