Spurious run-make test failure

[alexcrichton]

Example logs: http://buildbot.rust-lang.org/builders/auto-linux-64-opt-mir/builds/535/steps/test/logs/stdio

failures:

---- [run-make] run-make/alloc-extern-crates stdout ----
    thread '[run-make] run-make/alloc-extern-crates' panicked at 'failed to create dir "/buildslave/rust-buildbot/slave/auto-linux-64-opt-mir/build/obj/x86_64-unknown-linux-gnu/test/run-make/alloc-extern-crates.stage2-x86_64-unknown-linux-gnu": No such file or directory (os error 2)', /buildslave/rust-buildbot/slave/auto-linux-64-opt-mir/build/src/tools/compiletest/src/runtest.rs:2160
note: Run with `RUST_BACKTRACE=1` for a backtrace.


failures:
    [run-make] run-make/alloc-extern-crates

test result: FAILED. 125 passed; 1 failed; 0 ignored; 0 measured

Only instance I know of, but seems bad!

[alexcrichton]

Never saw this again, so assuming fixed