We got into problems since using `string.py` would shadow the string module from
the standard library. By some reason I adopted a pattern of `_` as suffix, but
let us just use the standard pattern of `test_` prefix like a normal testing
framework like pytest does.
This makes it easier to add a new test-case, and makes it easier to work with
the existing files. It does have a downside on making it a bit more annoying
looking at TestTaint.expected, and possible longer runtime, but I think it's
still worth it.
