html5ever-external-test.rs is now gone. Its custom test runner has moved
into tokenizer.rs and tree_builder.rs. We're using the #[start]
attribute to define our own entry point (#[main] isn't respected for
tests) which generates the test cases and calls through to
test::test_main.

tests/util.rs now lives in its own test_util crate. It's the only way I
could figure out to share this functionality between the tokenizer and
tree builder integration tests.

Makefile.in is now a little simpler because we can just use `cargo test`
to handle building the tests and examples and and running the tests.
Top-level make targets like `build`, `check`, and `examples` continue to
work (though `make examples` also builds the tests now).