tests/README.md uses confusing metasyntax

[danielshahaf]

tests/README.md speaks of $todo and $skip_test, even though the first is a metasyntactic variable and the second is a real variable. What's more, there's a literal $todo variable in the test harness implementation, which admittedly has nothing to do with the metasyntactic $todo in the documentation, but might confuse the maintainers nonetheless.

Let's use a different, less confusing notation.