Unit testing for SetNodeNameOrDie in package cmd/options

1. Why is this change necessary ?
fixes: kubernetes/node-problem-detector#161

2. How does this change address the issue ?
Under package cmd/options, the testing for SetNodeNameOrDie need
to decide Nodename based on environment variable "NODE_NAME" or
hostname or hostnameoverride variable.

3. How to verify this change ?
Run "go test" with admin privilege

Signed-off-by: gkGaneshR <gkganesh126@gmail.com>

cmd/options/options_test.go

@@ -0,0 +1,134 @@
+package options
+
+import (
+	"fmt"
+	"os"
+	"os/exec"
+	"testing"
+)
+
+type Options struct {
+	Nodename         string
+	Hostname         string
+	HostnameOverride string
+}
+
+//TestSetNodeNameOrDie tests for permutations of nodename, hostname and hostnameoverride
+func TestSetNodeNameOrDie(t *testing.T) {
+	options := map[string]struct {
+		Expected         Options
+		ObtainedNodeName string
+	}{
+		"Check Node and HostnameOverride only": {
+			Expected: Options{
+				Nodename:         "my-node-name",
+				Hostname:         "",
+				HostnameOverride: "override",
+			},
+		},
+		"Check Nodename only": {
+			Expected: Options{
+				Nodename:         "my-node-name",
+				Hostname:         "",
+				HostnameOverride: "",
+			},
+		},
+
+		"Check HostnameOverride only": {
+			Expected: Options{
+				Nodename:         "",
+				Hostname:         "",
+				HostnameOverride: "override",
+			},
+		},
+		"Check Hostname only": {
+			Expected: Options{
+				Nodename:         "",
+				Hostname:         "my-host-name",
+				HostnameOverride: "",
+			},
+		},
+		"Check Node, host and HostnameOverride only": {
+			Expected: Options{
+				Nodename:         "my-node-name",
+				Hostname:         "my-host-name",
+				HostnameOverride: "override",
+			},
+		},
+
+		"Check Host and HostnameOverride only": {
+			Expected: Options{
+				Nodename:         "",
+				Hostname:         "my-host-name",
+				HostnameOverride: "override",
+			},
+		},
+
+		"Check Node and hostname": {
+			Expected: Options{
+				Nodename:         "my-node-name",
+				Hostname:         "my-host-name",
+				HostnameOverride: "",
+			},
+		},
+	}
+
+	orig_node_name := os.Getenv("NODE_NAME")
+	orig_host_name, err := os.Hostname()
+	if err != nil {
+		fmt.Println("Unable to get hostname")
+	}
+
+	for str, opt := range options {
+		if opt.Expected.Nodename != "" {
+			err = os.Setenv("NODE_NAME", opt.Expected.Nodename)
+			if err != nil {
+				t.Errorf("Unable to set env NODE_NAME")
+			}
+		}
+
+		if opt.Expected.Hostname != "" {
+			//Changing hostname
+			cmd := exec.Command("hostname", opt.Expected.Hostname)
+			_, err := cmd.CombinedOutput()
+			if err != nil {
+				//If changing hostname requires admin privilege
+				cmd = exec.Command("sudo", "hostname", opt.Expected.Hostname)
+				_, err := cmd.CombinedOutput()
+				if err != nil {
+					fmt.Println("Unable to change hostname")
+					return
+				}
+			}
+		}
+
+		npdObj := NewNodeProblemDetectorOptions()
+		npdObj.HostnameOverride = opt.Expected.HostnameOverride
+		npdObj.SetNodeNameOrDie()
+		opt.ObtainedNodeName = npdObj.NodeName
+
+		if opt.ObtainedNodeName != opt.Expected.HostnameOverride &&
+			opt.ObtainedNodeName != opt.Expected.Nodename &&
+			opt.ObtainedNodeName != opt.Expected.Hostname {
+			t.Errorf("Error at : %+v", str)
+			t.Errorf("Wanted: %+v. \nGot: %+v", opt.Expected.Nodename, opt.ObtainedNodeName)
+		}
+
+		err = os.Setenv("NODE_NAME", "")
+		if err != nil {
+			t.Errorf("Unable to set env NODE_NAME empty")
+		}
+
+	}
+
+	err = os.Setenv("NODE_NAME", orig_node_name)
+	if err != nil {
+		fmt.Println("Unable to set original : env NODE_NAME")
+	}
+	cmd := exec.Command("sudo", "hostname", orig_host_name)
+	_, err = cmd.CombinedOutput()
+	if err != nil {
+		fmt.Println("Unable to set hostname")
+	}
+
+}