code: public AppValidator succeedsWith(String expectedMessage) {
          stringsToFind.add(expectedMessage);
          stringsToFind.add(APP_START_CODE);
          server.addIgnoredErrors(Collections.singletonList(expectedMessage));
          return this;
      }
docs: Specify that the app should start, but that the given message should be seen during app startup
  <p>
  This can be used to ensure that warning messages are emitted