Odoo Online (and Odoo.sh and Odoo On Premise) databases are only upgraded at the request of an authorized User.

For Odoo Online databases, you can review who requested the upgrade by visiting https://www.odoo.com/my/databases and checking the Access Log:

1) Right click on a database and select the View Access Log option:

2) Look for the entry action Request upgrade:

The following dialog is shown after an upgrade TEST has been performed:

It is not possible to bypass the creation of a TEST database.  

It is not possible for a database to be upgraded without the request coming from a database User.

None of this happens automatically, it has to be initiated by a User via the TEST UPGRADE button, then a test database is made available, THEN the option to CONFIRM THE UPGRADE is made available (and another Request upgrade action will be logged).