prefetch_canceled randomly failed

[gaul]

Seems racy. Travis logs:

---- prefetch_canceled stdout ----
1392thread 'prefetch_canceled' panicked at 'assertion failed: `(left == right)`
1393  left: `"f"`,
1394 right: `"file1\n"`', tests/integration_tests.rs:516:13
1395stack backtrace:
1396   0: backtrace::backtrace::libunwind::trace
1397             at /cargo/registry/src/github.com-1ecc6299db9ec823/backtrace-0.3.40/src/backtrace/libunwind.rs:88
1398   1: backtrace::backtrace::trace_unsynchronized
1399             at /cargo/registry/src/github.com-1ecc6299db9ec823/backtrace-0.3.40/src/backtrace/mod.rs:66
1400   2: std::sys_common::backtrace::_print_fmt
1401             at src/libstd/sys_common/backtrace.rs:77
1402   3: <std::sys_common::backtrace::_print::DisplayBacktrace as core::fmt::Display>::fmt
1403             at src/libstd/sys_common/backtrace.rs:59
1404   4: core::fmt::write
1405             at src/libcore/fmt/mod.rs:1052
1406   5: std::io::Write::write_fmt
1407             at /rustc/b8cedc00407a4c56a3bda1ed605c6fc166655447/src/libstd/io/mod.rs:1426
1408   6: std::io::impls::<impl std::io::Write for alloc::boxed::Box<W>>::write_fmt
1409             at src/libstd/io/impls.rs:156
1410   7: std::sys_common::backtrace::_print
1411             at src/libstd/sys_common/backtrace.rs:62
1412   8: std::sys_common::backtrace::print
1413             at src/libstd/sys_common/backtrace.rs:49
1414   9: std::panicking::default_hook::{{closure}}
1415             at src/libstd/panicking.rs:204
1416  10: std::panicking::default_hook
1417             at src/libstd/panicking.rs:221
1418  11: std::panicking::rust_panic_with_hook
1419             at src/libstd/panicking.rs:472
1420  12: rust_begin_unwind
1421             at src/libstd/panicking.rs:380
1422  13: std::panicking::begin_panic_fmt
1423             at src/libstd/panicking.rs:334
1424  14: integration_tests::prefetch_canceled
1425             at tests/integration_tests.rs:516
1426  15: integration_tests::prefetch_canceled::{{closure}}
1427             at tests/test_suite/mod.rs:6
1428  16: core::ops::function::FnOnce::call_once
1429             at /rustc/b8cedc00407a4c56a3bda1ed605c6fc166655447/src/libcore/ops/function.rs:232
1430  17: <alloc::boxed::Box<F> as core::ops::function::FnOnce<A>>::call_once
1431             at /rustc/b8cedc00407a4c56a3bda1ed605c6fc166655447/src/liballoc/boxed.rs:1015
1432  18: __rust_maybe_catch_panic
1433             at src/libpanic_unwind/lib.rs:86
1434  19: std::panicking::try
1435             at /rustc/b8cedc00407a4c56a3bda1ed605c6fc166655447/src/libstd/panicking.rs:281
1436  20: std::panic::catch_unwind
1437             at /rustc/b8cedc00407a4c56a3bda1ed605c6fc166655447/src/libstd/panic.rs:394
1438  21: test::run_test_in_process
1439             at src/libtest/lib.rs:539
1440  22: test::run_test::run_test_inner::{{closure}}
1441             at src/libtest/lib.rs:452