Add two unit tests for our toml util module.
- Write an object with util.toml, read it with util.toml, and compare
written and read objects.
- Write an object directly as a string, read it with util.toml,
comparing with an expected object.
A test that writes with util.toml, reads as string, and verifies the
read string is difficult to do in a general way, because each toml
module we support writes files in a slightly different way.
