tests fail under rust 1.77.1 to build because of rustc-test

[rillian]

cargo test fails to build under rust 1.77.1 because of the rustc-test transitive dependency on rustc-serialize. Looks like this is already reported upstream, but I expect the whole crate is no longer necessary and the dependency can removed.

[rillian]

The inclusion in xml5ever isn't used; it looks like the depending code was moved to markup5ever in 7e87faf.

It's still used in rcdom as a harness for running tests based on runtime discovery of test suite files. There are newer crates which support this, and rustc-test is also a servo project, so that could also be updated. But since it's testing the public api, I suggest just replacing it with some logging.