code: function createOlmSession(olmAccount, recipientTestClient) {
      return recipientTestClient.awaitOneTimeKeyUpload().then((keys) => {
          const otkId = utils.keys(keys)[0];
          const otk = keys[otkId];
  
          const session = new global.Olm.Session();
          session.create_outbound(
              olmAccount, recipientTestClient.getDeviceKey(), otk.key,
          );
          return session;
      });
  }
docs: start an Olm session with a given recipient
  
  @param {Olm.Account} olmAccount
  @param {TestClient} recipientTestClient
  @return {Promise} promise for Olm.Session