rustdoc --test interleaves failure output

[huonw]

Seems like it's because it's not capturing stderr.

#[crate_id="par-rustdoc"];

/*!
```
?
```

```
!
```

```
$
```
*/

<anon>:6:1: 6:2 <anon>:5:1: 5:2error:  unexpected token: `}`
error: unexpected token: `$`
<anon>:<anon>6: 5} 
$
                  ^
^
<anon>:5:1: 5:2 error: unknown start of token: ?
<anon>:5 ?
         ^
test _0 ... FAILED
test _2 ... FAILED
test _1 ... FAILED

failures:

---- _0 stdout ----
    task '_0' failed at '~Any', /home/huon/rust/src/libsyntax/diagnostic.rs:49

---- _2 stdout ----
    task '_2' failed at '~Any', /home/huon/rust/src/libsyntax/diagnostic.rs:49

---- _1 stdout ----
    task '_1' failed at '~Any', /home/huon/rust/src/libsyntax/diagnostic.rs:49


failures:
    _0
    _1
    _2

test result: FAILED. 0 passed; 3 failed; 0 ignored; 0 measured

[huonw]

cc @cmr, @alexcrichton