std: Ignore a flaky std::comm test

This test relies on the parent to be descheduled before the child sends its
data. This has proved to be unreliable on libnative on the bots. It's a fairly
trivial test regardless, so ignoring it for now won't lose much.
auto merge of #13356 : alexcrichton/rust/ignore-flaky, r=huonw

This test relies on the parent to be descheduled before the child sends its
data. This has proved to be unreliable on libnative on the bots. It's a fairly
trivial test regardless, so ignoring it for now won't lose much.