private static String getServices(String documentType) {
        return "<services version='1.0'>" +
               "  <content id='test' version='1.0'>" +
               "    <redundancy>1</redundancy>" +
               "    <engine>" +
               "    <proton/>" +
               "    </engine>" +
               "    <documents>" +
               "      <document type='" + documentType + "' mode='index'/>" +
               "    </documents>" +
               "    <nodes count='1'/>" +
               "   </content>" +
               "</services>";
    }
Tests validation of removal of a document type.
Uses the types "music" and "book" as those are supported by the ValidationTester.

@author bratseth
config-model/src/test/java/com/yahoo/vespa/model/application/validation/change/ContentTypeRemovalValidatorTest.java