The link of testing will be https://play.google.com/apps/testing/packagename and you can find that in Alpha/Beta publish tab.
https://play.google.com/apps/publish --> Select app --> Release Management --> App Release --> Alpha/Beta --> Manage --> Opt-in URL (URL for testing)
Updated to add tester (Close test)
Add testers
- On the left menu, select Settings Settings > Manage testers > Create
a list.
- Type a name to identify your list of testers. You can use the same list for future tests on any of your apps.
- Add email addresses and create list.
- On the left menu,select Release management > App releases.
- Next, to the closed track, select Manage.
- Next to "Choose a testing method," select Closed Testing.
- In the "Users" section, check the box next to the list you want to use.
- List of will be notified and selected user can accept the invitation and test the app.