How to install Windows Phone app for beta testing without purchasing a certificate

I am a newbie to WP app development. I have a Windows Phone individual developer account and need to do a beta testing internally. I used Appaloosa and HockeyApp for this purpose. But, when I try to download the app and install it on my Windows Phone, it shows a message "Can't install company app" on both Appaloosa and HockeyApp.

I heard that one needs to have a Symantec certificate and an .aetx file in order to install Windows Phone app which costs $299.00.

I need to know if there is any other option so that I can install my developed app freely on some Windows Phone devices which are intended for testing. I might need to email my .xap file to the testers and let them download and install for testing purpose.

Thanks in advance.