Auto merge of #6561 - ehuss:fix-switch-features, r=alexcrichton

Fix spurious Windows errors with switch_features_rerun.

Fix a flaky test on Windows which fails with "Access is denied.".

Previous errors:
https://ci.appveyor.com/project/rust-lang-libs/cargo/builds/17766698
https://ci.appveyor.com/project/rust-lang-libs/cargo/builds/18040797
https://ci.appveyor.com/project/rust-lang-libs/cargo/builds/20472056
https://ci.appveyor.com/project/rust-lang-libs/cargo/builds/18746372
https://ci.appveyor.com/project/rust-lang-libs/cargo/builds/18559776
https://ci.appveyor.com/project/rust-lang-libs/cargo/builds/19659571
https://ci.appveyor.com/project/rust-lang-libs/cargo/builds/18956349
https://ci.appveyor.com/project/rust-lang-libs/cargo/builds/18183191
https://ci.appveyor.com/project/rust-lang-libs/cargo/builds/20334485
https://ci.appveyor.com/project/rust-lang-libs/cargo/builds/18554058

diff --cc tests/testsuite/support/mod.rs
Simple merge