- add an invalid xml example

resources/test/invalid/minimod.xml

+<?xml version="1.0" encoding="utf-8"?>
+<mods:mods xmlns:mods="http://www.loc.gov/mods/v3"
+  xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
+  xsi:schemaLocation="http://www.loc.gov/mods/v3 https://www.loc.gov/standards/mods/v3/mods-3-8.xsd">
+  <mods:identer>123</mods:identer>
+</mods:mods>