Fundamental: A test is not a unit test if it touches the file system. A NOT TO DO EXAMPLE in Go.

myfile.go

package myfile

// openFileInPath Opens a file in the fileLocation path.
// It return the file pointer or any error encountered.
func openFile(fileLocation string) (*os.File, error) {
	filePointer, err := os.Open(fileLocation)
	if err != nil {
		return nil, err
	}

	return filePointer, nil
}

myfile_test.go

package myfile

func TestOpenFileFileExistsInPath(t *testing.T) {
	licenseFile := "/tmp/test-file.txt"
	licenceFileContent := []byte("This is the key. Must ignore other lines.\nMust be ignored.\n Must be ignored.\n")

	err := ioutil.WriteFile(licenseFile, licenceFileContent, 0400)
	if err != nil {
		t.Errorf(err.Error())
	}

	t.Run("", func(t *testing.T) {
		filePointer, _ := openFile(licenseFile)
		filePointerType := fmt.Sprint(reflect.TypeOf(filePointer))

		if strings.Compare(filePointerType, "*os.File") != 0 {
			t.Fatal("Error while reading file in the path")
		}

		kindOfFile := reflect.ValueOf(filePointer).Kind()

		if kindOfFile != reflect.Ptr {
			t.Fatal("Error while reading file in the path")
		}
	})

	err = os.Remove(licenseFile)
	if err != nil {
		fmt.Println(err)
		return
	}
}

The better way to do is in case of need to do would be with Mocks.
Mocks are what we are talking about here: objects pre-programmed with expectations which form a specification of the calls they are expected to receive.