Loading 10,000 users is the same as loading 1 user. Just map the fields appropriately and upload the records via Data Loader/API/etc. To answer your questions:
What are the steps/or how can we load 10,000 users to salesforce org?
Install Apex Data Loader, create CSV file, upload users to target org.
What kind of permissions or salesforce editions needed to test?
Manage Users and Modify All Data (e.g. a System Administrator), and any Salesforce Edition that has API access (e.g. Enterprise or higher).
Any recommended open source test tools?
There's literally no reason to test this. Salesforce is more than capable of creating 10,000 users, and it's no more challenging than creating a single user.
If you can successfully upload one record, you can successfully upload 10,000 records. You're not going to find a "test" environment that has 10,000 licenses available, short of using one of the client's sandboxes. Even then, you're really wasting your time.
Again, loading 10,000 users is exactly as difficult as loading 1 user. You simply need to make sure that all your data is in order.