More mysterious coretest races.

[bblum]

Exposed with:

while ./x86_64-unknown-linux-gnu/test/coretest.stage0-x86_64-unknown-linux-gnu;
do true; done

• coretest run::tests::test_pipes fails nondeterministically. No failure message, it seems... it just... fails.
• Can hang (all schedulers idle) after a very long time (15-30min?).

[brson]

I still see coretest time out sometimes. Won't be fixed in 0.6.

[catamorphism]

@brson Is this still happening?

[brson]

Haven't seen any deadlocks or nondeterministic pipe failures recently.