Add test id for languageReferenceButton

Signed-off-by: Jialiang Liang <jiallian@amazon.com>

src/plugins/data/public/query/query_string/language_service/lib/language_reference.tsx

@@ -37,6 +37,7 @@ export const LanguageReference = (props: {
           defaultMessage: `Language Reference`,
         })}
         onClick={() => setIsLanguageReferenceOpen(!isLanguageReferenceOpen)}
+        data-test-subj="languageReferenceButton"
       />
     </div>
   );