Add unit test

Author: lirannl

tests/allow_missing.rs

@@ -0,0 +1,11 @@
+use rust_embed::Embed;
+
+#[derive(Embed)]
+#[folder = "examples/missing/"]
+#[allow_missing = true]
+struct Asset;
+
+#[test]
+fn missing_is_empty() {
+  assert_eq!(Asset::iter().count(), 0);
+}