Add test for disk input validation

builder/azure/chroot/builder_test.go

@@ -0,0 +1,28 @@
+package chroot
+
+import (
+	"regexp"
+	"testing"
+
+	"github.com/hashicorp/packer/packer"
+)
+
+func TestBuilder_Prepare_DiskAsInput(t *testing.T) {
+	b := Builder{}
+	_, err := b.Prepare(map[string]interface{}{
+		"source": "/subscriptions/28279221-ccbe-40f0-b70b-4d78ab822e09/resourceGroups/testrg/providers/Microsoft.Compute/disks/diskname",
+	})
+
+	if err != nil {
+		// make sure there is no error about the source field
+		errs, ok := err.(*packer.MultiError)
+		if !ok {
+			t.Error("Expected the returned error to be of type packer.MultiError")
+		}
+		for _, err := range errs.Errors {
+			if matched, _:=regexp.MatchString(`(^|\W)source\W`,err.Error()); matched {
+				t.Errorf("Did not expect an error about the 'source' field, but found %q", err)
+			}
+		}
+	}
+}