Is looking up the source of tests and using those to practice considered cheating?
No, that is not cheating as the comments point out. You have invested time looking for material to practise and come across some. However, if you had to “hack” a system to get the test that is a different case.... :)

The “cheating” or , at least poor practice is from the teacher just taking and using verbatim a test from somewhere - editing, changing questions, changing numbers is often done to make a new or suitable version.

Using a test from somewhere else may be ok, but all the questions need to be checked : vocabulary, level etc before use and adjusted as necessary.