Is there a way to copy my current site collection in order to use that copy as a testing environment in SharePoint Online?
Unfortunately, there is not a simple method available in SharePoint Online to copy an entire site collection in SharePoint online. The only method available in SharePoint Online is to do a manual copy/backup for each site under the site collection using the Save Site as Template including its content feature (Include Content).
read the response from the MSFT official over here
Other thing, if you are using any 3rd party tool for backup / restore then you can use it.
For SharePoint Online - the way to do this is via PnP :-
Get-PnPProvisioningTemplate -Url -Out to get the provisioning Template Apply-PnPProvisioningTemplate -Path to Apply the template to the new site / Site Collection
This will get all of the site columns, site content types, term store, list definitions, (empty) lists, Site Structure and copy them to the new site.
If you want content moved across as well, then that needs more PowerShell, but is doable.