If your testing requires pulling quasi-real credit reports from the bureaus, the inactive SSNs of other answers won't work and you'll need designated test numbers.
I found this site Which appears to contain test social security numbers with associated test names and credit card numbers.
Transunion has a test environment you can link and send data to, including associated dummy credit reports. Sending a SSN to them with certain numbers in certain positions will automatically route the inquiry to their test environment Other credit bureaus will have similar systems in place.