oci/auth: Add test to check for non-test flags

Add a black box test to import the auth package as a consumer of the
package and make sure that no flags are injected. Being in a test, it
ignores all the default test flags with "test." prefix.

Signed-off-by: Sunny <darkowlzz@protonmail.com>

oci/auth/flag_test.go

@@ -0,0 +1,36 @@
+/*
+Copyright 2023 The Flux 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.
+See the License for the specific language governing permissions and
+limitations under the License.
+*/
+
+// Make sure we never inject non-test flags when the auth packages are imported.
+// Refer https://github.com/fluxcd/pkg/issues/645.
+package auth_test
+
+import (
+	"flag"
+	"strings"
+	"testing"
+
+	_ "github.com/fluxcd/pkg/oci/auth/login"
+)
+
+func TestNonTestFlagCheck(t *testing.T) {
+	flagCheck := func(f *flag.Flag) {
+		if !strings.HasPrefix(f.Name, "test.") {
+			t.Errorf("found non-test command line flag: %q", f.Name)
+		}
+	}
+	flag.VisitAll(flagCheck)
+}