The SQL standard requires the engine to return a "not found" status when the user ID is not the owner of the database and privileges have not been granted to the user ID (see the GRANT statement).
Both the interpreter and the test program take one argument, which is the user ID. The user ID defaults to PUBLIC. However, PUBLIC should not own any tables. If you run the interpreter with no argument, the tables are owned by PUBLIC and are thus not accessible.
The trick is to run the interpreter with an argument, which is the user ID, and then to run BigTest with the same argument. The argument can be any user ID you choose, conforming to the ANSI/ODBC standard for SQL names: 1-18 characters, beginning with a letter and consisting of letters and numbers.
In version 8, the user ID is hard coded into bigt.cxx. You may simply use that user ID (SETH) when creating the test database (with the newstuff script), or you may edit bigt.cxx to change the user ID.