Added tests for the nodeportlocal package

This commit introduces new tests for the `pkg/agent/nodeportlocal/rules`
package.

Fixes: #5650

Signed-off-by: VaibhavMalik4187 <vaibhavmalik2018@gmail.com>

pkg/agent/nodeportlocal/rules/iptable_rule_test.go

@@ -0,0 +1,49 @@
+// Copyright 2024 Antrea 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.
+
+package rules
+
+import (
+	"testing"
+
+	"github.com/stretchr/testify/require"
+)
+
+func TestInitAndAddRule(t *testing.T) {
+	rules := InitRules()
+	require.NotNil(t, rules)
+
+	err := rules.Init()
+	require.ErrorContains(t, err, "initialization of NPL iptables rules failed")
+
+	err = rules.AddRule(7, "", 77, "tcp")
+	require.ErrorContains(t, err, "error checking if rule [-p")
+
+	nodePortRules := []PodNodePort{
+		{
+			Protocols: []string{"tcp"},
+		},
+	}
+	expectedErr := "error executing iptables-restore:"
+
+	err = rules.AddAllRules(nodePortRules)
+
+	require.ErrorContains(t, err, expectedErr)
+
+	err = rules.DeleteRule(7, "", 77, "tcp")
+	require.ErrorContains(t, err, "error checking if rule [-p")
+
+	err = rules.DeleteAllRules()
+	require.ErrorContains(t, err, "failed to check if NodePortLocal chain exists in NAT table:")
+}