Auto merge of #3021 - alexcrichton:test-release-panic-abort, r=brson

Fix transitive doctests panic=abort

Ensure that when we compile doctested libraries or examples we use the same
panic mode as the rest of the tests, namely ignoring panic=abort b/c libtest
isn't compiled with panic=abort.

Closes #3017

diff --cc src/cargo/ops/cargo_compile.rs
Simple merge