message: Add sensible missing entry generator for MELS tests
  Fixes vector-im/riot-web#5426 (because we don't test plurals anywhere else)
diff: @@ -88,6 +88,9 @@ describe('MemberEventListSummary', function() {
  sandbox = testUtils.stubClient();
  languageHandler.setLanguage('en').done(done);
  + languageHandler.setMissingEntryGenerator(function(key) {
  + return key.split('|', 2)[1];
  + });
  });
  afterEach(function() {
  