Split overlays into discrete functionality and test

- Closes #987

To make the overlays testable in isolation, and after discussion with
the TSC, removed host/time/source from overlay files.

Signed-off-by: Jonathon Anderson <janderson@ciq.com>

overlays/debian.interfaces/internal/debian_interfaces_test.go

@@ -0,0 +1,73 @@
+package debian_interfaces
+
+import (
+	"bytes"
+	"testing"
+
+	"github.com/stretchr/testify/assert"
+	"github.com/warewulf/warewulf/internal/app/wwctl/overlay/show"
+	"github.com/warewulf/warewulf/internal/pkg/testenv"
+	"github.com/warewulf/warewulf/internal/pkg/wwlog"
+)
+
+func Test_wickedOverlay(t *testing.T) {
+	env := testenv.New(t)
+	defer env.RemoveAll(t)
+	env.ImportFile(t, "etc/warewulf/nodes.conf", "nodes.conf")
+	env.ImportFile(t, "var/lib/warewulf/overlays/debian.interfaces/rootfs/etc/network/interfaces.d/default.ww", "../rootfs/etc/network/interfaces.d/default.ww")
+
+	tests := []struct {
+		name string
+		args []string
+		log  string
+	}{
+		{
+			name: "debian.interfaces",
+			args: []string{"--render", "node1", "debian.interfaces", "etc/network/interfaces.d/default.ww"},
+			log:  debian_interfaces,
+		},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			cmd := show.GetCommand()
+			cmd.SetArgs(tt.args)
+			stdout := bytes.NewBufferString("")
+			stderr := bytes.NewBufferString("")
+			logbuf := bytes.NewBufferString("")
+			cmd.SetOut(stdout)
+			cmd.SetErr(stderr)
+			wwlog.SetLogWriter(logbuf)
+			err := cmd.Execute()
+			assert.NoError(t, err)
+			assert.Empty(t, stdout.String())
+			assert.Empty(t, stderr.String())
+			assert.Equal(t, tt.log, logbuf.String())
+		})
+	}
+}
+
+const debian_interfaces string = `backupFile: true
+writeFile: true
+Filename: default
+# This file is autogenerated by warewulf
+auto wwnet0
+allow-hotplug wwnet0
+iface wwnet0 inet static
+  address 192.168.3.21
+  netmask 255.255.255.0
+  gateway 192.168.3.1
+  mtu 
+backupFile: true
+writeFile: true
+Filename: secondary
+# This file is autogenerated by warewulf
+auto wwnet1
+allow-hotplug wwnet1
+iface wwnet1 inet static
+  address 192.168.3.22
+  netmask 255.255.255.0
+  gateway 192.168.3.1
+  mtu 
+  up ifmetric wwnet1 30
+`