Add test case for OUT_DIR in tests.

[sayrer]

This doesn't pass right now, but it does pass under cargo test. I see the rustc rule has a way to pass OUT_DIR, but it looks like tests aren't getting this right now. Posting this now, before I dig further, in case someone knows of a quick fix here.

It looks like this cargo behavior is intentional: rust-lang/cargo#879

[sayrer]

I decided it's probably not worth it to emulate cargo here, after thinking about it some more.

This patch should probably land for test coverage and documentation purposes. I searched the repo for OUT_DIR while trying to figure out how to make it work in integration tests.

[hlopko]

I'm a bit confused, is the test supposed to fail with bazel at the moment? Because the CI seems to be green. Do we have a bug in the CI configuration?

[sayrer]

I'm a bit confused, is the test supposed to fail with bazel at the moment? Because the CI seems to be green. Do we have a bug in the CI configuration?

No--it all works, it's just undocumented (although implied by rust_test kwargs) and uncovered by tests, I think. I had to write this PR to figure it out. This PR tests passing OUT_DIR to integration tests at compile time, and shows that Bazel does not set OUT_DIR at test runtime. Cargo does set OUT_DIR at runtime, but that happened by mistake, and they are having trouble undoing this behavior now (the docs still insist OUT_DIR is only available at compile time).

[hlopko]

Overall it's a nice PR, thank you! To me it makes sense to check it in. Only one nit remaining.