Use `.tst` directory to keep tst data

[Dalton Serey]

Every tst object directory should contain a .tst hidden directory within it. Regular files should be kept visible within the tst object directory. While all specific tst files should be kept within the .tst hidden directory. The point is to make it clearer who owns each file, and how does the user access and modify data within them. This is a much safer way to keep files the user is not supposed to deal with directly.