add new unit tests in auth_handler_test.go

internal/testutil/string_slice.go

@@ -0,0 +1,14 @@
+// Copyright 2023 the Pinniped contributors. All Rights Reserved.
+// SPDX-License-Identifier: Apache-2.0
+
+package testutil
+
+import "fmt"
+
+func AddPrefixToEach(prefix string, addToEach []string) []string {
+	result := make([]string, len(addToEach))
+	for i, s := range addToEach {
+		result[i] = fmt.Sprintf("%s%s", prefix, s)
+	}
+	return result
+}