Allow message specification for should_fail

The test harness will make sure that the panic message contains the
specified string. This is useful to help make `#[should_fail]` tests a
bit less brittle by decreasing the chance that the test isn't
"accidentally" passing due to a panic occurring earlier than expected.
The behavior is in some ways similar to JUnit's `expected` feature:
`@Test(expected=NullPointerException.class)`.

Without the message assertion, this test would pass even though it's not
actually reaching the intended part of the code:
```rust
fn test_oob_array_access() {
    let idx: uint = from_str("13o").unwrap(); // oops, this will panic
    [1i32, 2, 3][idx];
}
```