According to the site, the Census Bureau has produced test data from the ACS since 1996. However, I was not able to find the data from 1996 to 1999.
The ACS test data for years prior to 2000 is not an “official” ACS sample and does not have full coverage of the entire population. IPUMS USA does not provide access to these files; we provide access to ACS samples from 2000 onward. The Census Bureau does provide access if you email your name and mailing address to firstname.lastname@example.org.