Auto merge of #23718 - alexcrichton:flaky-test, r=huonw

It's considered an error to access stdout while a process is being shut down, so
tweak this test a bit to actually wait for the child thread to exit.

This was discovered with a recent [snap-mac3 failure](http://buildbot.rust-lang.org/builders/snap3-mac/builds/164/steps/test/logs/stdio)