A "Test", or "Sandbox" is used in order to simulate the provisioning workflows without incurring any costs to the vendor. Typically, Google does not offer a "sandbox environments" for Google Workspace Services, however, you can create a "Test Reseller Account" which requires the manual conversion of a "normal/standard" account to a "test" account, by Google.
On this page, you can view the process that needs to be followed before Google creates a Test Reseller Account.
Creating a New Account
As an existing Google Partner/Reseller you need to sign up for a NEW direct trial account. Then Google can convert this to a test reseller account.
Important: Domain verification is required. Make sure DNS settings are accessible.
Once you have done so, you need to OPEN A SUPPORT TICKET via the Reseller portal:
… (or send an email to Google Partner Support at pps-support@google.com), asking Google Support to convert this new account (you need to specify the test domain) to a Test Reseller Account.