Testing inapp subscription billing is horrible. You can test out of your SDK for the adroid.test.purchased to make sure you are parsing responses properly. To test the subscription, add a test product to the subscription products on the developer console and publish the product. You must publish the products for this to work. Get up google groups and add test email accounts to the group. Invite the testers. THEN upload your .apk to an ALPHA and publish it. you must publish it as an alpha for the test phones and logins to find it.
Go to the link provided under the ALPHA tab to set up testers and log in from the phone using one of the test emails. Buy the inapp product to make sure all your code works. You can cancel the test product purchases in the Google Wallet and re-use the products until the code is correct. THEN set up a test subscription product. You will only be able to test the subscription once per test email. There is no way to remove the test subscription. Be sure to change the product names in your apk before you upload. Also, change the version code in the AndroidManifest, and turn off debugging.
If you try to install the app twice on a device, you may need to clear the data from these apps under settings in the phone: chrome, play store, play services, or it will continue to think you have it installed and won't download it. You must access the test .apk from a browser, NOT through the play store.
This is the most onerous testing I have ever experienced. Also, watch for errors from Google IABilling response, since the "already owned" response for a product is different for a subscription. I am still trying to figure out what the response is for the already owned subscription. Will update if I find out. This explanation is the ONLY information available anywhere that I have found, and this was weeks of trial and error. If you have more data on this, please share!!!