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.
I took the bits from ql/test/library-tests/taint/ that seemed easy to port. I
left out namedtuple for now, but it is part of internal tracking ticket, so
won't be forgotten.
