check playht can fetch voices when adding new speech credential

lib/utils/speech-utils.js

@@ -256,6 +256,14 @@ const testPlayHT = async(logger, synthAudio, credentials) => {
         text: 'Hi there and welcome to jambones!'
       }
     );
+    // Test if playHT can fetch voices
+    const get = bent('https://api.play.ht', 'GET', 'json', {
+      'AUTHORIZATION' : credentials.api_key,
+      'X-USER-ID': credentials.user_id,
+      'Accept': 'application/json'
+    });
+
+    await Promise.all([ get('/api/v2/cloned-voices'), get('/api/v2/voices')]);
   } catch (err) {
     logger.info({err}, 'synth Playht returned error');
     throw err;