extra: Capture stdout/stderr of tests by default

[alexcrichton]

When tests fail, their stdout and stderr is printed as part of the summary, but
this helps suppress failure messages from #[should_fail] tests and generally
clean up the output of the test runner.

[alexcrichton]

With this test:

#[test]
fn test1() {
    fail!()
}
#[test]
fn test2() {
}
#[test]
#[should_fail]
fn test3() {
    fail!()
}
#[test]
#[should_fail]
fn test4() {
}

BEFORE

running 4 tests
task 'test1' failed at 'explicit failure', foo.rs:3
task 'test3' failed at 'explicit failure', foo.rs:11
test test2 ... ok
test test4 ... FAILED
test test1 ... FAILED
test test3 ... ok

failures:
    test1
    test4

test result: FAILED. 2 passed; 2 failed; 0 ignored; 0 measured

task '<main>' failed at 'Some tests failed', /Users/alex/code/rust3/src/libextra/test.rs:167

AFTER

running 4 tests
test test2 ... ok
test test1 ... FAILED
test test3 ... ok
test test4 ... FAILED

failures:

---- test1 stdout ----
        task 'test1' failed at 'explicit failure', foo.rs:3


failures:
    test1
    test4

test result: FAILED. 2 passed; 2 failed; 0 ignored; 0 measured

task '<main>' failed at 'Some tests failed', /Users/alex/code/rust-opt/src/libextra/test.rs:166

[sfackler]

from_utf8_lossy seems like it'd make more sense here.

[alexcrichton]

Updated with from_utf8_lossy

[adrientetar]

👌