The PCT test MsBuildBuilderTest#configRoundtrip fails on Jenkins > 2.266 because of an old HtmlUnit with:
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com.gargoylesoftware.htmlunit.ScriptException: TypeError: Cannot find function createHTMLDocument in object [object DOMImplementation]. (http://localhost:38087/jenkins/static/ad451fd8/jsbundles/vendors.js#25)          

 The real failure is swallowed by:

Not valid configuration for MsBuild

Because the AssertionError doesn't append the root cause:
 
https://github.com/jenkinsci/msbuild-plugin/blob/f566e248dd59bfcbf3125f7eeda064da16b15c6b/src/test/java/hudson/plugins/msbuild/MsBuildBuilderTest.java#L57
 
This ticket is to fix the PCT failure by upgrading dependencies to grab a newer HtmlUnit and to improve the assertions by adding the root cause.