tests/unit.rs broken on nightly builds

[behnam]

Travis: https://travis-ci.org/servo/rust-url/jobs/248944414

Example error:

error[E0560]: struct `__test::test::TestDesc` has no field named `allow_fail`
  --> tests/unit.rs:20:1
   |
20 | / fn size() {
21 | |     use std::mem::size_of;
22 | |     assert_eq!(size_of::<Url>(), size_of::<Option<Url>>());
23 | | }
   | |_^ `__test::test::TestDesc` does not have this field

I think it's caused by tests/unit.rs having test harness enabled, and the test internals having changed and not conflicting rustc-test package used here.

I'm not sure what's the best option for a fix:

• Disabling harness for unit.rs and copying in boilerplate to get the tests running; or
• Updating rustc-test to make it compatible.

What do you think, @SimonSapin ?

[SimonSapin]

I think we should make rustc-test use https://crates.io/crates/rustc_version to conditionally compile and work with compiler that do or do not include the allow_fail field.