Auto merge of #2232 - rust-lang:gesundheit, r=oli-obk
Auto merge of #2232 - rust-lang:gesundheit, r=oli-obk

Always show stderr on test failure.

fixes #2224

I overengineered the original thing to the point where it became fragile. Let's just always print stderr, unless it was already printed

rust-lang/miri,rust-lang/miri,rust-lang/miri,rust-lang/miri